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Chapter 7

SUMMARY OF THE HEIGHTS AND WEIGHTS OF THE CROSSED AND SELF-FERTILISED PLANTS.


Number of species and plants measured.
Tables given.
Preliminary remarks on the offspring of plants crossed by a fresh stock.
Thirteen cases specially considered.
The effects of crossing a self-fertilised plant either by another self-fertilised plant or by an intercrossed plant of the
old stock.
Summary of the results.
Preliminary remarks on the crossed and self-fertilised plants of the same stock.
The twenty-six exceptional cases considered, in which the crossed plants did not exceed greatly in height the self-fertilised.
Most of these cases shown not to be real exceptions to the rule that cross-fertilisation is beneficial.
Summary of results.
Relative weights of the crossed and self-fertilised plants.


The details which have been given under the head of each species are so numerous and so intricate, that it is necessary to tabulate the results. In Table 7/A, the number of plants of each kind which were raised from a cross between two individuals of the same stock and from self-fertilised seeds, together with their mean or average heights, are given. In the right hand column, the mean height of the crossed to that of the self-fertilised plants, the former being taken as 100, is shown. To make this clear, it may be advisable to give an example. In the first generation of Ipomoea, six plants derived from a cross between two plants were measured, and their mean height is 86.00 inches; six plants derived from flowers on the same parent-plant fertilised with their own pollen were measured, and their mean height is 65.66 inches. From this it follows, as shown in the right hand column, that if the mean height of the crossed plants be taken as 100, that of the self-fertilised plants is 76. The same plan is followed with all the other species.

The crossed and self-fertilised plants were generally grown in pots in competition with one another, and always under as closely similar conditions as could be attained. They were, however, sometimes grown in separate rows in the open ground. With several of the species, the crossed plants were again crossed, and the self-fertilised plants again self-fertilised, and thus successive generations were raised and measured, as may be seen in Table 7/A. Owing to this manner of proceeding, the crossed plants became in the later generations more or less closely inter-related.

In Table 7/B the relative weights of the crossed and self-fertilised plants, after they had flowered and had been cut down, are given in the few cases in which they were ascertained. The results are, I think, more striking and of greater value as evidence of constitutional vigour than those deduced from the relative heights of the plants.

The most important table is Table 7/C, as it includes the relative heights, weights, and fertility of plants raised from parents crossed by a fresh stock (that is, by non-related plants grown under different conditions), or by a distinct sub-variety, in comparison with self-fertilised plants, or in a few cases with plants of the same old stock intercrossed during several generations. The relative fertility of the plants in this and the other tables will be more fully considered in a future chapter.

TABLE 7/A. Relative heights of plants from parents crossed with pollen from other plants of the same stock, and self-fertilised.

Heights of plants measured in inches.

Column 1: Name of Plant.

Column 2: Number of Crossed Plants measured.

Column 3: Average Height of Crossed Plants.

Column 4: Number of Self-fertilised Plants measured.

Column 5: Average Height of Self-fertilised Plants.

Column 6: x, where the ratio of the Average Height of the Crossed to the Self-fertilised Plants is expressed as 100 to x.


Ipomoea purpurea--first generation: 6 : 86.00 : 6 : 65.66 : 76.

Ipomoea purpurea--second generation: 6 : 84.16 : 6 : 66.33 : 79.

Ipomoea purpurea--third generation: 6 : 77.41 : 6 : 52.83 : 68.

Ipomoea purpurea--fourth generation: 7 : 69.78 : 7 : 60.14 : 86.

Ipomoea purpurea--fifth generation: 6 : 82.54 : 6 : 62.33 : 75.

Ipomoea purpurea--sixth generation: 6 : 87.50 : 6 : 63.16 : 72.

Ipomoea purpurea--seventh generation: 9 : 83.94 : 9 : 68.25 : 81.

Ipomoea purpurea--eighth generation: 8 : 113.25 : 8 : 96.65 : 85.

Ipomoea purpurea--ninth generation: 14 : 81.39 : 14 : 64.07 : 79.

Ipomoea purpurea--tenth generation: 5 : 93.70 : 5 : 50.40 : 54.

Ipomoea purpurea--Number and average height of all the plants of the ten generations: 73 : 85.84 : 73 : 66.02 : 77.

Mimulus luteus--three first generations, before the new and taller self-fertilised variety appeared: 10 : 8.19 : 10 : 5.29 : 65.

Digitalis purpurea: 16 : 51.33 : 8 : 35.87 : 70.

Calceolaria--(common greenhouse variety): 1 : 19.50 : 1 : 15.00 : 77.

Linaria vulgaris: 3 : 7.08 : 3 : 5.75 : 81.

Verbascum thapsus: 6 : 65.34 : 6 : 56.50 : 86.

Vandellia nummularifolia--crossed and self-fertilised plants, raised from perfect flowers: 20 : 4.30 : 20 : 4.27 : 99.

Vandellia nummularifolia--crossed and self-fertilised plants, raised from perfect flowers: second trial, plants crowded: 24 : 3.60 : 24 : 3.38 : 94.

Vandellia nummularifolia--crossed plants raised from perfect flowers, and self-fertilised plants from cleistogene flowers: 20 : 4.30 : 20 : 4.06 : 94.

Gesneria pendulina: 8 : 32.06 : 8 : 29.14 : 90.

Salvia coccinea: 6 : 27.85 : 6 : 21.16 : 76.

Origanum vulgare: 4 : 20.00 : 4 : 17.12 : 86.

Thunbergia alata: 6 : 60.00 : 6 : 65.00 : 108.

Brassica oleracea: 9 : 41.08 : 9 : 39.00 : 95.

Iberis umbellata--the self-fertilised plants of the third generation: 7 : 19.12 : 7 : 16.39 : 86.

Papaver vagum: 15 : 21.91 : 15 : 19.54 : 89.

Eschscholtzia californica--English stock, first generation: 4 : 29.68 : 4 : 25.56 : 86.

Eschscholtzia californica--English stock, second generation: 11 : 32.47 : 11 : 32.81 : 101.

Eschscholtzia californica--Brazilian stock, first generation: 14 : 44.64 : 14 : 45.12 : 101.

Eschscholtzia californica--Brazilian stock, second generation: 18 : 43.38 : 19 : 50.30 : 116.

Eschscholtzia californica--average height and number of all the plants of Eschscholtzia: 47 : 40.03 : 48 : 42.72 : 107.

Reseda lutea--grown in pots: 24 : 17.17 : 24 : 14.61 : 85.

Reseda lutea--grown in open ground : 8 : 28.09 : 8 : 23.14 : 82.

Reseda odorata--self-fertilised seeds from a highly self-fertile plant, grown in pots: 19 : 27.48 : 19 : 22.55 : 82.

Reseda odorata--self-fertilised seeds from a highly self-fertile plant, grown in open ground: 8 : 25.76 : 8 : 27.09 : 105.

Reseda odorata--self-fertilised seeds from a semi-self-fertile plant, grown in pots: 20 : 29.98 : 20 : 27.71 : 92.

Reseda odorata--self-fertilised seeds from a semi-self-fertile plant, grown in open ground: 8 : 25.92 : 8 : 23.54 : 90.

Viola tricolor: 14 : 5.58 : 14 : 2.37 : 42.

Adonis aestivalis: 4 : 14.25 : 4 : 14.31 : 100.

Delphinium consolida: 6 : 14.95 : 6 : 12.50 : 84.

Viscaria oculata: 15 : 34.50 : 15 : 33.55 : 97.

Dianthus caryophyllus--open ground, about : 6?: 28? : 6?: 24? : 86.

Dianthus caryophyllus--second generation, in pots, crowded: 2 : 16.75 : 2 : 9.75 : 58.

Dianthus caryophyllus--third generation, in pots: 8 : 28.39 : 8 : 28.21 : 99.

Dianthus caryophyllus--offspring from plants of the third self-fertilised generation crossed by intercrossed plants of the third generation, compared with plants of fourth self-fertilised generation: 15 : 28.00 : 10 : 26.55 : 95.

Dianthus caryophyllus--number and average height of all the plants of Dianthus: 31 : 27.37 : 26 : 25.18 : 92.

Hibiscus africanus: 4 : 13.25 : 4 : 14.43 : 109.

Pelargonium zonale: 7 : 22.35 : 7 : 16.62 : 74.

Tropaeolum minus: 8 : 58.43 : 8 : 46.00 : 79.

Limnanthes douglasii: 16 : 17.46 : 16 : 13.85 : 79.

Lupinus luteus--second generation: 8 : 30.78 : 8 : 25.21 : 82.

Lupinus pilosus--plants of two generations: 2 : 35.50 : 3 : 30.50 : 86.

Phaseolus multiflorus: 5 : 86.00 : 5 : 82.35 : 96.

Pisum sativum: 4 : 34.62 : 4 : 39.68 : 115.

Sarothamnus scoparius--small seedlings: 6 : 2.91 : 6 : 1.33 : 46.

Sarothamnus scoparius--the three survivors on each side after three years' growth: : 18.91 : : 11.83 : 63.

Ononis minutissima: 2 : 19.81 : 2 : 17.37 : 88.

Clarkia elegans: 4 : 33.50 : 4 : 27.62 : 82.

Bartonia aurea: 8 : 24.62 : 8 : 26.31 : 107.

Passiflora gracilis: 2 : 49.00 : 2 : 51.00 : 104.

Apium petroselinum: * : : * : : 100. *not measured.

Scabiosa atro-purpurea: 4 : 17.12 : 4 : 15.37 : 90.

Lactuca sativa--plants of two generations: 7 : 19.43 : 6 : 16.00 : 82.

Specularia speculum: 4 : 19.28 : 4 : 18.93 : 98.

Lobelia ramosa--first generation: 4 : 22.25 : 4 : 18.37 : 82.

Lobelia ramosa--second generation: 3 : 23.33 : 3 : 19.00 : 81.

Lobelia fulgens--first generation: 2 : 34.75 : 2 : 44.25 : 127.

Lobelia fulgens--second generation: 23 : 29.82 : 23 : 27.10 : 91.

Nemophila insignis--half-grown: 12 : 11.10 : 12 : 5.45 : 49.

Nemophila insignis--the same fully-grown: : 33.28 : : 19.90 : 60.

Borago officinalis: 4 : 20.68 : 4 : 21.18 : 102.

Nolana prostrata: 5 : 12.75 : 5 : 13.40 : 105.

Petunia violacea--first generation: 5 : 30.80 : 5 : 26.00 : 84.

Petunia violacea--second generation: 4 : 40.50 : 6 : 26.25 : 65.

Petunia violacea--third generation: 8 : 40.96 : 8 : 53.87 : 131.

Petunia violacea--fourth generation: 15 : 46.79 : 14 : 32.39 : 69.

Petunia violacea--fourth generation, from a distinct parent: 13 : 44.74 : 13 : 26.87 : 60.

Petunia violacea--fifth generation: 22 : 54.11 : 21 : 33.23 : 61.

Petunia violacea--fifth generation, in open ground: 10 : 38.27 : 10 : 23.31 : 61.

Petunia violacea--Number and average height of all the plants in pots of Petunia: 67 : 46.53 : 67 : 33.12 : 71.

Nicotiana tabacum--first generation: 4 : 18.50 : 4 : 32.75 : 178.

Nicotiana tabacum--second generation: 9 : 53.84 : 7 : 51.78 : 96.

Nicotiana tabacum--third generation: 7 : 95.25 : 7 : 79.60 : 83.

Nicotiana tabacum--third generation but raised from a distinct plant: 7 : 70.78 : 9 : 71.30 : 101.

Nicotiana tabacum--Number and average height of all the plants of Nicotiana: 27 : 63.73 : 27 : 61.31 : 96.

Cyclamen persicum: 8 : 9.49 : 8?: 7.50 : 79.

Anagallis collina: 6 : 42.20 : 6 : 33.35 : 69.

Primula sinensis--a dimorphic species: 8 : 9.01 : 8 : 9.03 : 100.

Fagopyrum esculentum--a dimorphic species: 15 : 38.06 : 15 : 26.13 : 69.

Beta vulgaris--in pots: 8 : 34.09 : 8 : 29.81 : 87.

Beta vulgaris--in open ground: 8 : 30.92 : 8 : 30.70 : 99.

Canna warscewiczi--plants of three generations: 34 : 35.98 : 34 : 36.39 : 101.

Zea mays--in pots, whilst young, measured to tips of leaves: 15 : 20.19 : 15 : 17.57 : 87.

Zea mays--when full-grown, after the death of some, measured to tips of leaves: : 68.10 : : 62.34 : 91.

Zea mays--when full-grown, after the death of some, measured to tips of flowers: : 66.51 : : 61.59 : 93.

Zea mays--grown in open ground, measured to tips of leaves: 10 : 54.00 : 10 : 44.55 : 83.

Zea mays--grown in open ground, measured to tips of flowers: : 53.96 : : 43.45 : 80.

Phalaris canariensis--in pots. 11 : 38.90 : 11 : 35.69 : 92.

Phalaris canariensis--in open ground: 12 : 35.78 : 12 : 33.50 : 93.

TABLE 7/B.--Relative weights of plants from parents crossed with pollen from distinct plants of the same stock, and self-fertilised.

Column 1: Names of plants.

Column 2: Number of crossed plants.

Column 3: Number of self-fertilised plants.

Column 4: x, where the ratio of the Weight of the Crossed to the Self-fertilised Plants is expressed as 100 to x.

Ipomoea purpurea--plants of the tenth generation: 6 : 6 : 44.

Vandellia nummularifolia--first generation: 41 : 41 : 97.

Brassica oleracea--first generation: 9 : 9 : 37.

Eschscholtzia californica--plants of the second generation: 19 : 19 : 118.

Reseda lutea--first generation, grown in pots: 24 : 24 : 21.

Reseda lutea--first generation, grown in open ground: 8 : 8 : 40.

Reseda odorata--first generation, descended from a highly self-fertile plant, grown in pots: 19 : 19 : 67.

Reseda odorata--first generation, descended from a semi-self-fertile plant, grown in pots: 20 : 20 : 99.

Dianthus caryophyllus--plants of the third generation: 8 : 8 : 49.

Petunia violacea--plants of the fifth generation, in pots: 22 : 21 : 22.

Petunia violacea--plants of the fifth generation, in open ground: 10 : 10 : 36.

TABLE 7/C.--Relative heights, weights, and fertility of plants from parents crossed by a fresh stock, and from parents either self-fertilised or intercrossed with plants of the same stock.

Column 1: Names of the plants and nature of the experiments.

Column 2: Number of plants from a cross with a fresh stock.

Column 3: Average height in inches and weight.

Column 4: Number of the plants from self-fertilised or intercrossed parents of the same stock.

Column 5: Average height in inches and weight.

Column 4: x, where the ratio of the Height, Weight and Fertility of the plants from the Cross with a fresh stock is expressed as 100 to x.

Ipomoea purpurea--offspring of plants intercrossed for nine generations and then crossed by a fresh stock, compared with plants of the tenth intercrossed generation: 19 : 84.03 : 19 : 65.78 : 78.

Ipomoea purpurea--offspring of plants intercrossed for nine generations and then crossed by a fresh stock, compared with plants of the tenth intercrossed generation, in fertility: .. : .. : .. : .. : 51.

Mimulus luteus--offspring of plants self-fertilised for eight generations and then crossed by a fresh stock, compared with plants of the ninth self-fertilised generation: 28 : 21.62 : 19 : 10.44 : 52.

Mimulus luteus--offspring of plants self-fertilised for eight generations and then crossed by a fresh stock, compared with plants of the ninth self-fertilised generation, in fertility: .. : .. : .. : .. : 3.

Mimulus luteus--offspring of plants self-fertilised for eight generations and then crossed by a fresh stock, compared with the offspring of a plant self-fertilised for eight generations, and then intercrossed with another self-fertilised plant of the same generation: 28 : 21.62 : 27 : 12.20 : 56.

Mimulus luteus--offspring of plants self-fertilised for eight generations and then crossed by a fresh stock, compared with the offspring of a plant self-fertilised for eight generations, and then intercrossed with another self-fertilised plant of the same generation, in fertility: .. : .. : .. : .. : 4.

Brassica oleracea--offspring of plants self-fertilised for two generations and then crossed by a fresh stock, compared with plants of the third self-fertilised generation, by weight: 6 : : 6 : : 22.

Iberis umbellata--offspring from English variety crossed by slightly different Algerine variety, compared with the self-fertilised offspring of the English variety: 30 : 17.34 : 29 : 15.51 : 89.

Iberis umbellata--offspring from English variety crossed by slightly different Algerine variety, compared with the self-fertilised offspring of the English variety, in fertility: .. : .. : .. : .. : 75.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an English stock, compared with plants of the Brazilian stock of the second self-fertilised generation: 19 : 45.92 : 19 : 50.30 : 109.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an English stock, compared with plants of the Brazilian stock of the second self-fertilised generation, in weight: .. : .. : .. : .. : 118.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an English stock, compared with plants of the Brazilian stock of the second self-fertilised generation, in fertility: .. : .. : .. : .. : 40.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an English stock, compared with plants of the Brazilian stock of the second intercrossed generation, in height: 19 : 45.92 : 18 : 43.38 : 94.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an English stock, compared with plants of the Brazilian stock of the second intercrossed generation, in weight: .. : .. : .. : .. : 100.

Eschscholtzia californica--offspring of a Brazilian stock crossed by an English stock, compared with plants of the Brazilian stock of the second intercrossed generation, in fertility: .. : .. : .. : .. : 45.

Dianthus caryophyllus--offspring of plants self-fertilised for three generations and then crossed by a fresh stock, compared with plants of the fourth self-fertilised generation: 16 : 32.82 : 10 : 26.55 : 81.

Dianthus caryophyllus--offspring of plants self-fertilised for three generations and then crossed by a fresh stock, compared with plants of the fourth self-fertilised generation, in fertility: .. : .. : .. : .. : 33.

Dianthus caryophyllus--offspring of plants self-fertilised for three generations and then crossed by a fresh stock, compared with the offspring of plants self-fertilised for three generations and then crossed by plants of the third intercrossed generation: 16 : 32.82 : 15 : 28.00 : 85.

Dianthus caryophyllus--offspring of plants self-fertilised for three generations and then crossed by a fresh stock, compared with the offspring of plants self-fertilised for three generations and then crossed by plants of the third intercrossed generation, in fertility: .. : .. : .. : .. : 45.

Pisum sativum--offspring from a cross between two closely allied varieties, compared with the self-fertilised offspring of one of the varieties, or with intercrossed plants of the same stock: ? : : ? : : 60 to 75.

Lathyrus odoratus--offspring from two varieties, differing only in colour of their flowers, compared with the self-fertilised offspring of one of the varieties: in first generation: 2 : 79.25 : 2 : 63.75 : 80.

Lathyrus odoratus--offspring from two varieties, differing only in colour of their flowers, compared with the self-fertilised offspring of one of the varieties: in second generation: 6 : 62.91 : 6 : 55.31 : 88.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth self-fertilised generation, in height: 21 : 50.05 : 21 : 33.23 : 66.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth self-fertilised generation, in weight: .. : .. : .. : .. : 23.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth self-fertilised generation, grown in open ground, in height: 10 : 36.67 : 10 : 23.31 : 63.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth self-fertilised generation, grown in open ground, in weight: .. : .. : .. : .. : 53.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth self-fertilised generation, grown in open ground, in fertility: .. : .. : .. : .. : 46.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth intercrossed generation, in height: 21 : 50.05 : 22 : 54.11 : 108.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth intercrossed generation, in weight: .. : .. : .. : .. : 101.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth intercrossed generation, grown in open ground, in height: 10 : 36.67 : 10 : 38.27 : 104.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth intercrossed generation, grown in open ground, in weight: .. : .. : .. : .. : 146.

Petunia violacea--offspring of plants self-fertilised for four generations and then crossed by a fresh stock, compared with plants of the fifth intercrossed generation, grown in open ground, in fertility: .. : .. : .. : .. : 54.

Nicotiana tabacum--offspring of plants self-fertilised for three generations and then crossed by a slightly different variety, compared with plants of the fourth self-fertilised generation, grown not much crowded in pots, in height: 26 : 63.29 : 26 : 41.67 : 66.

Nicotiana tabacum--offspring of plants self-fertilised for three generations and then crossed by a slightly different variety, compared with plants of the fourth self-fertilised generation, grown much crowded in pots, in height: 12 : 31.53 : 12 : 17.21 : 54.

Nicotiana tabacum--offspring of plants self-fertilised for three generations and then crossed by a slightly different variety, compared with plants of the fourth self-fertilised generation, grown much crowded in pots, in weight: .. : .. : .. : .. : 37.

Nicotiana tabacum--offspring of plants self-fertilised for three generations and then crossed by a slightly different variety, compared with plants of the fourth self-fertilised generation, grown in open ground, in height: 20 : 48.74 : 20 : 35.20 : 72.

Nicotiana tabacum--offspring of plants self-fertilised for three generations and then crossed by a slightly different variety, compared with plants of the fourth self-fertilised generation, grown in open ground, in weight: .. : .. : .. : .. : 63.

Anagallis collina--offspring from a red variety crossed by a blue variety, compared with the self-fertilised offspring of the red variety: 3 : 27.62 : 3 : 18.21 : 66.

Anagallis collina--offspring from a red variety crossed by a blue variety, compared with the self-fertilised offspring of the red variety, in fertility: .. : .. : .. : .. : 6.

Primula veris--offspring from long-styled plants of the third illegitimate generation, crossed by a fresh stock, compared with plants of the fourth illegitimate and self-fertilised generation: 8 : 7.03 : 8 : 3.21 : 46.

Primula veris--offspring from long-styled plants of the third illegitimate generation, crossed by a fresh stock, compared with plants of the fourth illegitimate and self-fertilised generation, in fertility: .. : .. : .. : .. : 5.

Primula veris--offspring from long-styled plants of the third illegitimate generation, crossed by a fresh stock, compared with plants of the fourth illegitimate and self-fertilised generation, in fertility in following year: .. : .. : .. : .. : 3.5.

Primula veris--(equal-styled, red-flowered variety)--offspring from plants self-fertilised for two generations and then crossed by a different variety, compared with plants of the third self-fertilised generation: 3 : 8.66 : 3 : 7.33 : 85.

Primula veris--(equal-styled, red-flowered variety)--offspring from plants self-fertilised for two generations and then crossed by a different variety, compared with plants of the third self-fertilised generation, in fertility: .. : .. : .. : .. : 11.


In these three tables the measurements of fifty-seven species, belonging to fifty-two genera and to thirty great natural families, are given. The species are natives of various parts of the world. The number of crossed plants, including those derived from a cross between plants of the same stock and of two different stocks, amounts to 1,101; and the number of self-fertilised plants (including a few in Table 7/C derived from a cross between plants of the same old stock) is 1,076. Their growth was observed from the germination of the seeds to maturity; and most of them were measured twice and some thrice. The various precautions taken to prevent either lot being unduly favoured, have been described in the introductory chapter. Bearing all these circumstances in mind, it may be admitted that we have a fair basis for judging of the comparative effects of cross-fertilisation and of self-fertilisation on the growth of the offspring.

It will be the most convenient plan first to consider the results given in Table 7/C, as an opportunity will thus be afforded of incidentally discussing some important points. If the reader will look down the right hand column of this table, he will see at a glance what an extraordinary advantage in height, weight, and fertility the plants derived from a cross with a fresh stock or with another sub-variety have over the self-fertilised plants, as well as over the intercrossed plants of the same old stock. There are only two exceptions to this rule, and these are hardly real ones. In the case of Eschscholtzia, the advantage is confined to fertility. In that of Petunia, though the plants derived from a cross with a fresh stock had an immense superiority in height, weight, and fertility over the self-fertilised plants, they were conquered by the intercrossed plants of the same old stock in height and weight, but not in fertility. It has, however, been shown that the superiority of these intercrossed plants in height and weight was in all probability not real; for if the two sets had been allowed to grow for another month, it is almost certain that those from a cross with the fresh stock would have been victorious in every way over the intercrossed plants.

Before we consider in detail the several cases given in Table 7/C, some preliminary remarks must be made. There is the clearest evidence, as we shall presently see, that the advantage of a cross depends wholly on the plants differing somewhat in constitution; and that the disadvantages of self-fertilisation depend on the two parents, which are combined in the same hermaphrodite flower, having a closely similar constitution. A certain amount of differentiation in the sexual elements seems indispensable for the full fertility of the parents, and for the full vigour of the offspring. All the individuals of the same species, even those produced in a state of nature, differ somewhat, though often very slightly, from one another in external characters and probably in constitution. This obviously holds good between the varieties of the same species, as far as external characters are concerned; and much evidence could be advanced with respect to their generally differing somewhat in constitution. There can hardly be a doubt that the differences of all kinds between the individuals and varieties of the same species depend largely, and as I believe exclusively, on their progenitors having been subjected to different conditions; though the conditions to which the individuals of the same species are exposed in a state of nature often falsely appear to us the same. For instance, the individuals growing together are necessarily exposed to the same climate, and they seem to us at first sight to be subjected to identically the same conditions; but this can hardly be the case, except under the unusual contingency of each individual being surrounded by other kinds of plants in exactly the same proportional numbers. For the surrounding plants absorb different amounts of various substances from the soil, and thus greatly affect the nourishment and even the life of the individuals of any particular species. These will also be shaded and otherwise affected by the nature of the surrounding plants. Moreover, seeds often lie dormant in the ground, and those which germinate during any one year will often have been matured during very different seasons. Seeds are widely dispersed by various means, and some will occasionally be brought from distant stations, where their parents have grown under somewhat different conditions, and the plants produced from such seeds will intercross with the old residents, thus mingling their constitutional peculiarities in all sorts of proportions.

Plants when first subjected to culture, even in their native country, cannot fail to be exposed to greatly changed conditions of life, more especially from growing in cleared ground, and from not having to compete with many or any surrounding plants. They are thus enabled to absorb whatever they require which the soil may contain. Fresh seeds are often brought from distant gardens, where the parent-plants have been subjected to different conditions. Cultivated plants like those in a state of nature frequently intercross, and will thus mingle their constitutional peculiarities. On the other hand, as long as the individuals of any species are cultivated in the same garden, they will apparently be subjected to more uniform conditions than plants in a state of nature, as the individuals have not to compete with various surrounding species. The seeds sown at the same time in a garden have generally been matured during the same season and in the same place; and in this respect they differ much from the seeds sown by the hand of nature. Some exotic plants are not frequented by the native insects in their new home, and therefore are not intercrossed; and this appears to be a highly important factor in the individuals acquiring uniformity of constitution.

In my experiments the greatest care was taken that in each generation all the crossed and self-fertilised plants should be subjected to the same conditions. Not that the conditions were absolutely the same, for the more vigorous individuals will have robbed the weaker ones of nutriment, and likewise of water when the soil in the pots was becoming dry; and both lots at one end of the pot will have received a little more light than those at the other end. In the successive generations, the plants were subjected to somewhat different conditions, for the seasons necessarily varied, and they were sometimes raised at different periods of the year. But as they were all kept under glass, they were exposed to far less abrupt and great changes of temperature and moisture than are plants growing out of doors. With respect to the intercrossed plants, their first parents, which were not related, would almost certainly have differed somewhat in constitution; and such constitutional peculiarities would be variously mingled in each succeeding intercrossed generation, being sometimes augmented, but more commonly neutralised in a greater or less degree, and sometimes revived through reversion; just as we know to be the case with the external characters of crossed species and varieties. With the plants which were self-fertilised during the successive generations, this latter important source of some diversity of constitution will have been wholly eliminated; and the sexual elements produced by the same flower must have been developed under as nearly the same conditions as it is possible to conceive.

In Table 7/C the crossed plants are the offspring of a cross with a fresh stock, or with a distinct variety; and they were put into competition either with self-fertilised plants, or with intercrossed plants of the same old stock. By the term fresh stock I mean a non-related plant, the progenitors of which have been raised during some generations in another garden, and have consequently been exposed to somewhat different conditions. In the case of Nicotiana, Iberis, the red variety of Primula, the common Pea, and perhaps Anagallis, the plants which were crossed may be ranked as distinct varieties or sub-varieties of the same species; but with Ipomoea, Mimulus, Dianthus, and Petunia, the plants which were crossed differed exclusively in the tint of their flowers; and as a large proportion of the plants raised from the same lot of purchased seeds thus varied, the differences may be estimated as merely individual. Having made these preliminary remarks, we will now consider in detail the several cases given in Table 7/C, and they are well worthy of full consideration.

1. Ipomoea purpurea.

Plants growing in the same pots, and subjected in each generation to the same conditions, were intercrossed for nine consecutive generations. These intercrossed plants thus became in the later generations more or less closely inter-related. Flowers on the plants of the ninth intercrossed generation were fertilised with pollen taken from a fresh stock, and seedlings thus raised. Other flowers on the same intercrossed plants were fertilised with pollen from another intercrossed plant, producing seedlings of the tenth intercrossed generation. These two sets of seedlings were grown in competition with one another, and differed greatly in height and fertility. For the offspring from the cross with a fresh stock exceeded in height the intercrossed plants in the ratio of 100 to 78; and this is nearly the same excess which the intercrossed had over the self-fertilised plants in all ten generations taken together, namely, as 100 to 77. The plants raised from the cross with a fresh stock were also greatly superior in fertility to the intercrossed, namely, in the ratio of 100 to 51, as judged by the relative weight of the seed-capsules produced by an equal number of plants of the two sets, both having been left to be naturally fertilised. It should be especially observed that none of the plants of either lot were the product of self-fertilisation. On the contrary, the intercrossed plants had certainly been crossed for the last ten generations, and probably, during all previous generations, as we may infer from the structure of the flowers and from the frequency of the visits of humble-bees. And so it will have been with the parent-plants of the fresh stock. The whole great difference in height and fertility between the two lots must be attributed to the one being the product of a cross with pollen from a fresh stock, and the other of a cross between plants of the same old stock.

This species offers another interesting case. In the five first generations in which intercrossed and self-fertilised plants were put into competition with one another, every single intercrossed plant beat its self-fertilised antagonist, except in one instance, in which they were equal in height. But in the sixth generation a plant appeared, named by me the Hero, remarkable for its tallness and increased self-fertility, and which transmitted its characters to the next three generations. The children of Hero were again self-fertilised, forming the eighth self-fertilised generation, and were likewise intercrossed one with another; but this cross between plants which had been subjected to the same conditions and had been self-fertilised during the seven previous generations, did not effect the least good; for the intercrossed grandchildren were actually shorter than the self-fertilised grandchildren, in the ratio of 100 to 107. We here see that the mere act of crossing two distinct plants does not by itself benefit the offspring. This case is almost the converse of that in the last paragraph, on which the offspring profited so greatly by a cross with a fresh stock. A similar trial was made with the descendants of Hero in the following generation, and with the same result. But the trial cannot be fully trusted, owing to the extremely unhealthy condition of the plants. Subject to this same serious cause of doubt, even a cross with a fresh stock did not benefit the great-grandchildren of Hero; and if this were really the case, it is the greatest anomaly observed by me in all my experiments.

2. Mimulus luteus.

During the three first generations the intercrossed plants taken together exceeded in height the self-fertilised taken together, in the ratio of 100 to 65, and in fertility in a still higher degree. In the fourth generation a new variety, which grew taller and had whiter and larger flowers than the old varieties, began to prevail, especially amongst the self-fertilised plants. This variety transmitted its characters with remarkable fidelity, so that all the plants in the later self-fertilised generations belonged to it. These consequently exceeded the intercrossed plants considerably in height. Thus in the seventh generation the intercrossed plants were to the self-fertilised in height as 100 to 137. It is a more remarkable fact that the self-fertilised plants of the sixth generation had become much more fertile than the intercrossed plants, judging by the number of capsules spontaneously produced, in the ratio of 147 to 100. This variety, which as we have seen appeared amongst the plants of the fourth self-fertilised generation, resembles in almost all its constitutional peculiarities the variety called Hero which appeared in the sixth self-fertilised generation of Ipomoea. No other such case, with the partial exception of that of Nicotiana, occurred in my experiments, carried on during eleven years.

Two plants of this variety of Mimulus, belonging to the sixth self-fertilised generation, and growing in separate pots, were intercrossed; and some flowers on the same plants were again self-fertilised. From the seeds thus obtained, plants derived from a cross between the self-fertilised plants, and others of the seventh self-fertilised generation, were raised. But this cross did not do the least good, the intercrossed plants being inferior in height to the self-fertilised, in the ratio of 100 to 110. This case is exactly parallel with that given under Ipomoea, of the grandchildren of Hero, and apparently of its great-grandchildren; for the seedlings raised by intercrossing these plants were not in any way superior to those of the corresponding generation raised from the self-fertilised flowers. Therefore in these several cases the crossing of plants, which had been self-fertilised for several generations and which had been cultivated all the time under as nearly as possible the same conditions, was not in the least beneficial.

Another experiment was now tried. Firstly, plants of the eighth self-fertilised generation were again self-fertilised, producing plants of the ninth self-fertilised generation. Secondly, two of the plants of the eighth self-fertilised generation were intercrossed one with another, as in the experiment above referred to; but this was now effected on plants which had been subjected to two additional generations of self-fertilisation. Thirdly, the same plants of the eighth self-fertilised generation were crossed with pollen from plants of a fresh stock brought from a distant garden. Numerous plants were raised from these three sets of seeds, and grown in competition with one another. The plants derived from a cross between the self-fertilised plants exceeded in height by a little the self-fertilised, namely, as 100 to 92; and in fertility in a greater degree, namely, as 100 to 73. I do not know whether this difference in the result, compared with that in the previous case, can be accounted for by the increased deterioration of the self-fertilised plants from two additional generations of self-fertilisation, and the consequent advantage of any cross whatever, along merely between the self-fertilised plants. But however this may be, the effects of crossing the self-fertilised plants of the eighth generation with a fresh stock were extremely striking; for the seedlings thus raised were to the self-fertilised of the ninth generation as 100 to 52 in height, and as 100 to 3 in fertility! They were also to the intercrossed plants (derived from crossing two of the self-fertilised plants of the eighth generation) in height as 100 to 56, and in fertility as 100 to 4. Better evidence could hardly be desired of the potent influence of a cross with a fresh stock on plants which had been self-fertilised for eight generations, and had been cultivated all the time under nearly uniform conditions, in comparison with plants self-fertilised for nine generations continuously, or then once intercrossed, namely in the last generation.

3. Brassica oleracea.

Some flowers on cabbage plants of the second self-fertilised generation were crossed with pollen from a plant of the same variety brought from a distant garden, and other flowers were again self-fertilised. Plants derived from a cross with a fresh stock and plants of the third self-fertilised generation were thus raised. The former were to the self-fertilised in weight as 100 to 22; and this enormous difference must be attributed in part to the beneficial effects of a cross with a fresh stock, and in part to the deteriorating effects of self-fertilisation continued during three generations.

4. Iberis umbellata.

Seedlings from a crimson English variety crossed by a pale-coloured variety which had been grown for some generations in Algiers, were to the self-fertilised seedlings from the crimson variety in height as 100 to 89, and as 100 to 75 in fertility. I am surprised that this cross with another variety did not produce a still more strongly marked beneficial effect; for some intercrossed plants of the crimson English variety, put into competition with plants of the same variety self-fertilised during three generations, were in height as 100 to 86, and in fertility as 100 to 75. The slightly greater difference in height in this latter case, may possibly be attributed to the deteriorating effects of self-fertilisation carried on for two additional generations.

5. Eschscholtzia californica.

This plant offers an almost unique case, inasmuch as the good effects of a cross are confined to the reproductive system. Intercrossed and self-fertilised plants of the English stock did not differ in height (nor in weight, as far as was ascertained) in any constant manner; the self-fertilised plants usually having the advantage. So it was with the offspring of plants of the Brazilian stock, tried in the same manner. The parent-plants, however, of the English stock produced many more seeds when fertilised with pollen from another plant than when self-fertilised; and in Brazil the parent-plants were absolutely sterile unless they were fertilised with pollen from another plant. Intercrossed seedlings, raised in England from the Brazilian stock, compared with self-fertilised seedlings of the corresponding second generation, yielded seeds in number as 100 to 89; both lots of plants being left freely exposed to the visits of insects. If we now turn to the effects of crossing plants of the Brazilian stock with pollen from the English stock,--so that plants which had been long exposed to very different conditions were intercrossed,--we find that the offspring were, as before, inferior in height and weight to the plants of the Brazilian stock after two generations of self-fertilisation, but were superior to them in the most marked manner in the number of seeds produced, namely, as 100 to 40; both lots of plants being left freely exposed to the visits of insects.

In the case of Ipomoea, we have seen that the plants derived from a cross with a fresh stock were superior in height as 100 to 78, and in fertility as 100 to 51, to the plants of the old stock, although these had been intercrossed during the last ten generations. With Eschscholtzia we have a nearly parallel case, but only as far as fertility is concerned, for the plants derived from a cross with a fresh stock were superior in fertility in the ratio of 100 to 45 to the Brazilian plants, which had been artificially intercrossed in England for the two last generations, and which must have been naturally intercrossed by insects during all previous generations in Brazil, where otherwise they are quite sterile.

6. Dianthus caryophyllus.

Plants self-fertilised for three generations were crossed with pollen from a fresh stock, and their offspring were grown in competition with plants of the fourth self-fertilised generation. The crossed plants thus obtained were to the self-fertilised in height as 100 to 81, and in fertility (both lots being left to be naturally fertilised by insects) as 100 to 33.

These same crossed plants were also to the offspring from the plants of the third generation crossed by the intercrossed plants of the corresponding generation, in height as 100 to 85, and in fertility as 100 to 45.

We thus see what a great advantage the offspring from a cross with a fresh stock had, not only over the self-fertilised plants of the fourth generation, but over the offspring from the self-fertilised plants of the third generation, when crossed by the intercrossed plants of the old stock.

7. Pisum sativum.

It has been shown under the head of this species, that the several varieties in this country almost invariably fertilise themselves, owing to insects rarely visiting the flowers; and as the plants have been long cultivated under nearly similar conditions, we can understand why a cross between two individuals of the same variety does not do the least good to the offspring either in height or fertility. This case is almost exactly parallel with that of Mimulus, or that of the Ipomoea named Hero; for in these two instances, crossing plants which had been self-fertilised for seven generations did not at all benefit the offspring. On the other hand, a cross between two varieties of the pea causes a marked superiority in the growth and vigour of the offspring, over the self-fertilised plants of the same varieties, as shown by two excellent observers. From my own observations (not made with great care) the offspring from crossed varieties were to self-fertilised plants in height, in one case as 100 to about 75, and in a second case as 100 to 60.

8. Lathyrus odoratus.

The sweet-pea is in the same state in regard to self-fertilisation as the common pea; and we have seen that seedlings from a cross between two varieties, which differed in no respect except in the colour of their flowers, were to the self-fertilised seedlings from the same mother-plant in height as 100 to 80; and in the second generation as 100 to 88. Unfortunately I did not ascertain whether crossing two plants of the same variety failed to produce any beneficial effect, but I venture to predict such would be the result.

9. Petunia violacea.

The intercrossed plants of the same stock in four out of the five successive generations plainly exceeded in height the self-fertilised plants. The latter in the fourth generation were crossed by a fresh stock, and the seedlings thus obtained were put into competition with the self-fertilised plants of the fifth generation. The crossed plants exceeded the self-fertilised in height in the ratio of 100 to 66, and in weight as 100 to 23; but this difference, though so great, is not much greater than that between the intercrossed plants of the same stock in comparison with the self-fertilised plants of the corresponding generation. This case, therefore, seems at first sight opposed to the rule that a cross with a fresh stock is much more beneficial than a cross between individuals of the same stock. But as with Eschscholtzia, the reproductive system was here chiefly benefited; for the plants raised from the cross with the fresh stock were to the self-fertilised plants in fertility, both lots being naturally fertilised, as 100 to 46, whereas the intercrossed plants of the same stock were to the self-fertilised plants of the corresponding fifth generation in fertility only as 100 to 86.

Although at the time of measurement the plants raised from the cross with the fresh stock did not exceed in height or weight the intercrossed plants of the old stock (owing to the growth of the former not having been completed, as explained under the head of this species), yet they exceeded the intercrossed plants in fertility in the ratio of 100 to 54. This fact is interesting, as it shows that plants self-fertilised for four generations and then crossed by a fresh stock, yielded seedlings which were nearly twice as fertile as those from plants of the same stock which had been intercrossed for the five previous generations. We here see, as with Eschscholtzia and Dianthus, that the mere act of crossing, independently of the state of the crossed plants, has little efficacy in giving increased fertility to the offspring. The same conclusion holds good, as we have already seen, in the analogous cases of Ipomoea, Mimulus, and Dianthus, with respect to height.

10. Nicotiana tabacum.

My plants were remarkably self-fertile, and the capsules from the self-fertilised flowers apparently yielded more seeds than those which were cross-fertilised. No insects were seen to visit the flowers in the hothouse, and I suspect that the stock on which I experimented had been raised under glass, and had been self-fertilised during several previous generations; if so, we can understand why, in the course of three generations, the crossed seedlings of the same stock did not uniformly exceed in height the self-fertilised seedlings. But the case is complicated by individual plants having different constitutions, so that some of the crossed and self-fertilised seedlings raised at the same time from the same parents behaved differently. However this may be, plants raised from self-fertilised plants of the third generation crossed by a slightly different sub-variety, exceeded greatly in height and weight the self-fertilised plants of the fourth generation; and the trial was made on a large scale. They exceeded them in height when grown in pots, and not much crowded, in the ratio of 100 to 66; and when much crowded, as 100 to 54. These crossed plants, when thus subjected to severe competition, also exceeded the self-fertilised in weight in the ratio of 100 to 37. So it was, but in a less degree (as may be seen in Table 7/C), when the two lots were grown out of doors and not subjected to any mutual competition. Nevertheless, strange as is the fact, the flowers on the mother-plants of the third self-fertilised generation did not yield more seed when they were crossed with pollen from plants of the fresh stock than when they were self-fertilised.

11. Anagallis collina.

Plants raised from a red variety crossed by another plant of the same variety were in height to the self-fertilised plants from the red variety as 100 to 73. When the flowers on the red variety were fertilised with pollen from a closely similar blue-flowered variety, they yielded double the number of seeds to what they did when crossed by pollen from another individual of the same red variety, and the seeds were much finer. The plants raised from this cross between the two varieties were to the self-fertilised seedlings from the red variety, in height as 100 to 66, and in fertility as 100 to 6.

12. Primula veris.

Some flowers on long-styled plants of the third illegitimate generation were legitimately crossed with pollen from a fresh stock, and others were fertilised with their own pollen. From the seeds thus produced crossed plants, and self-fertilised plants of the fourth illegitimate generation, were raised. The former were to the latter in height as 100 to 46, and in fertility during one year as 100 to 5, and as 100 to 3.5 during the next year. In this case, however, we have no means of distinguishing between the evil effects of illegitimate fertilisation continued during four generations (that is, by pollen of the same form, but taken from a distinct plant) and strict self-fertilisation. But it is probable that these two processes do not differ so essentially as at first appears to be the case. In the following experiment any doubt arising from illegitimate fertilisation was completely eliminated.

13. Primula veris. (Equal-styled, red-flowered variety.)

Flowers on plants of the second self-fertilised generation were crossed with pollen from a distinct variety or fresh stock, and others were again self-fertilised. Crossed plants and plants of the third self-fertilised generation, all of legitimate origin, were thus raised; and the former was to the latter in height as 100 to 85, and in fertility (as judged by the number of capsules produced, together with the average number of seeds) as 100 to 11.

SUMMARY OF THE MEASUREMENTS IN TABLE 7/C.

This table includes the heights and often the weights of 292 plants derived from a cross with a fresh stock, and of 305 plants, either of self-fertilised origin, or derived from an intercross between plants of the same stock. These 597 plants belong to thirteen species and twelve genera. The various precautions which were taken to ensure a fair comparison have already been stated. If we now look down the right hand column, in which the mean height, weight, and fertility of the plants derived from a cross with a fresh stock are represented by 100, we shall see by the other figures how wonderfully superior they are both to the self-fertilised and to the intercrossed plants of the same stock. With respect to height and weight, there are only two exceptions to the rule, namely, with Eschscholtzia and Petunia, and the latter is probably no real exception. Nor do these two species offer an exception in regard to fertility, for the plants derived from the cross with a fresh stock were much more fertile than the self-fertilised plants. The difference between the two sets of plants in the table is generally much greater in fertility than in height or weight. On the other hand, with some of the species, as with Nicotiana, there was no difference in fertility between the two sets, although a great difference in height and weight. Considering all the cases in this table, there can be no doubt that plants profit immensely, though in different ways, by a cross with a fresh stock or with a distinct sub-variety. It cannot be maintained that the benefit thus derived is due merely to the plants of the fresh stock being perfectly healthy, whilst those which had been long intercrossed or self-fertilised had become unhealthy; for in most cases there was no appearance of such unhealthiness, and we shall see under Table 7/A that the intercrossed plants of the same stock are generally superior to a certain extent to the self-fertilised,--both lots having been subjected to exactly the same conditions and being equally healthy or unhealthy.

We further learn from Table 7/C, that a cross between plants that have been self-fertilised during several successive generations and kept all the time under nearly uniform conditions, does not benefit the offspring in the least or only in a very slight degree. Mimulus and the descendants of Ipomoea named Hero offer instances of this rule. Again, plants self-fertilised during several generations profit only to a small extent by a cross with intercrossed plants of the same stock (as in the case of Dianthus), in comparison with the effects of a cross by a fresh stock. Plants of the same stock intercrossed during several generations (as with Petunia) were inferior in a marked manner in fertility to those derived from the corresponding self-fertilised plants crossed by a fresh stock. Lastly, certain plants which are regularly intercrossed by insects in a state of nature, and which were artificially crossed in each succeeding generation in the course of my experiments, so that they can never or most rarely have suffered any evil from self-fertilisation (as with Eschscholtzia and Ipomoea), nevertheless profited greatly by a cross with a fresh stock. These several cases taken together show us in the clearest manner that it is not the mere crossing of any two individuals which is beneficial to the offspring. The benefit thus derived depends on the plants which are united differing in some manner, and there can hardly be a doubt that it is in the constitution or nature of the sexual elements. Anyhow, it is certain that the differences are not of an external nature, for two plants which resemble each other as closely as the individuals of the same species ever do, profit in the plainest manner when intercrossed, if their progenitors have been exposed during several generations to different conditions. But to this latter subject I shall have to recur in a future chapter.

TABLE 7/A.

We will now turn to our first table, which relates to crossed and self-fertilised plants of the same stock. These consist of fifty-four species belonging to thirty natural orders. The total number of crossed plants of which measurements are given is 796, and of self-fertilised 809; that is altogether 1,605 plants. Some of the species were experimented on during several successive generations; and it should be borne in mind that in such cases the crossed plants in each generation were crossed with pollen from another crossed plant, and the flowers on the self-fertilised plants were almost always fertilised with their own pollen, though sometimes with pollen from other flowers on the same plant. The crossed plants thus became more or less closely inter-related in the later generations; and both lots were subjected in each generation to almost absolutely the same conditions, and to nearly the same conditions in the successive generations. It would have been a better plan in some respects if I had always crossed some flowers either on the self-fertilised or intercrossed plants of each generation with pollen from a non-related plant, grown under different conditions, as was done with the plants in Table 7/C; for by this procedure I should have learnt how much the offspring became deteriorated through continued self-fertilisation in the successive generations. As the case stands, the self-fertilised plants of the successive generations in Table 7/A were put into competition with and compared with intercrossed plants, which were probably deteriorated in some degree by being more or less inter-related and grown under similar conditions. Nevertheless, had I always followed the plan in Table 7/C, I should not have discovered the important fact that, although a cross between plants which are rather closely related and which had been subjected to closely similar conditions, gives during several generations some advantage to the offspring, yet that after a time they may be intercrossed with no advantage whatever to the offspring. Nor should I have learnt that the self-fertilised plants of the later generations might be crossed with intercrossed plants of the same stock with little or no advantage, although they profited to an extraordinary degree by a cross with a fresh stock.

With respect to the greater number of the plants in Table 7/A, nothing special need here be said; full particulars may be found under the head of each species by the aid of the Index. The figures in the right-hand column show the mean height of the self-fertilised plants, that of the crossed plants with which they competed being represented by 100. No notice is here taken of the few cases in which crossed and self-fertilised plants were grown in the open ground, so as not to compete together. The table includes, as we have seen, plants belonging to fifty-four species, but as some of these were measured during several successive generations, there are eighty-three cases in which crossed and self-fertilised plants were compared. As in each generation the number of plants which were measured (given in the table) was never very large and sometimes small, whenever in the right hand column the mean height of the crossed and self-fertilised plants is the same within five per cent, their heights may be considered as practically equal. Of such cases, that is, of self-fertilised plants of which the mean height is expressed by figures between 95 and 105, there are eighteen, either in some one or all the generations. There are eight cases in which the self-fertilised plants exceed the crossed by above five per cent, as shown by the figures in the right hand column being above 105. Lastly, there are fifty-seven cases in which the crossed plants exceed the self-fertilised in a ratio of at least 100 to 95, and generally in a much higher degree.

If the relative heights of the crossed and self-fertilised plants had been due to mere chance, there would have been about as many cases of self-fertilised plants exceeding the crossed in height by above five per cent as of the crossed thus exceeding the self-fertilised; but we see that of the latter there are fifty-seven cases, and of the former only eight cases; so that the cases in which the crossed plants exceed in height the self-fertilised in the above proportion are more than seven times as numerous as those in which the self-fertilised exceed the crossed in the same proportion. For our special purpose of comparing the powers of growth of crossed and self-fertilised plants, it may be said that in fifty-seven cases the crossed plants exceeded the self-fertilised by more than five per cent, and that in twenty-six cases (18 + 8) they did not thus exceed them. But we shall now show that in several of these twenty-six cases the crossed plants had a decided advantage over the self-fertilised in other respects, though not in height; that in other cases the mean heights are not trustworthy, owing to too few plants having been measured, or to their having grown unequally from being unhealthy, or to both causes combined. Nevertheless, as these cases are opposed to my general conclusion I have felt bound to give them. Lastly, the cause of the crossed plants having no advantage over the self-fertilised can be explained in some other cases. Thus a very small residue is left in which the self-fertilised plants appear, as far as my experiments serve, to be really equal or superior to the crossed plants.

We will now consider in some little detail the eighteen cases in which the self-fertilised plants equalled in average height the crossed plants within five per cent; and the eight cases in which the self-fertilised plants exceeded in average height the crossed plants by above five per cent; making altogether twenty-six cases in which the crossed plants were not taller than the self-fertilised plants in any marked degree.

[1. Dianthus caryophyllus (third generation).

This plant was experimented on during four generations, in three of which the crossed plants exceeded in height the self-fertilised generally by much more than five per cent; and we have seen under Table 7/C that the offspring from the plants of the third self-fertilised generation crossed by a fresh stock profited in height and fertility to an extraordinary degree. But in this third generation the crossed plants of the same stock were in height to the self-fertilised only as 100 to 99, that is, they were practically equal. Nevertheless, when the eight crossed and eight self-fertilised plants were cut down and weighed, the former were to the latter in weight as 100 to 49! There can therefore be not the least doubt that the crossed plants of this species are greatly superior in vigour and luxuriance to the self-fertilised; and what was the cause of the self-fertilised plants of the third generation, though so light and thin, growing up so as almost to equal the crossed in height, I cannot explain.

2. Lobelia fulgens (first generation).

The crossed plants of this generation were much inferior in height to the self-fertilised, in the proportion of 100 to 127. Although only two pairs were measured, which is obviously much too few to be trusted, yet from other evidence given under the head of this species, it is certain that the self-fertilised plants were very much more vigorous than the crossed. As I used pollen of unequal maturity for crossing and self-fertilising the parent-plants, it is possible that the great difference in the growth of their offspring may have been due to this cause. In the next generation this source of error was avoided, and many more plants were raised, and now the average height of the twenty-three crossed plants was to that of the twenty-three self-fertilised plants as 100 to 91. We can therefore hardly doubt that a cross is beneficial to this species.

3. Petunia violacea (third generation).

Eight crossed plants were to eight self-fertilised of the third generation in average height as 100 to 131; and at an early age the crossed were inferior even in a still higher degree. But it is a remarkable fact that in one pot in which plants of both lots grew extremely crowded, the crossed were thrice as tall as the self-fertilised. As in the two preceding and two succeeding generations, as well as with plants raised by a crossed with a fresh stock, the crossed greatly exceeded the self-fertilised in height, weight, and fertility (when these two latter points were attended to), the present case must be looked at as an anomaly not affecting the general rule. The most probable explanation is that the seeds from which the crossed plants of the third generation were raised were not well ripened; for I have observed an analogous case with Iberis. Self-fertilised seedlings of this latter plant, which were known to have been produced from seeds not well matured, grew from the first much more quickly than the crossed plants, which were raised from better matured seeds; so that having thus once got a great start they were enabled ever afterwards to retain their advantage. Some of these same seeds of the Iberis were sown on the opposite sides of pots filled with burnt earth and pure sand, not containing any organic matter; and now the young crossed seedlings grew during their short life to double the height of the self-fertilised, in the same manner as occurred with the above two sets of seedlings of Petunia which were much crowded and thus exposed to very unfavourable conditions. We have seen also in the eighth generation of Ipomoea that the self-fertilised seedlings raised from unhealthy parents grew at first very much more quickly than the crossed seedlings, so that they were for a long time much taller, though ultimately beaten by them.

4, 5, 6. Eschscholtzia californica.

Four sets of measurements are given in Table 7/A. In one of these the crossed plants exceed the self-fertilised in average height, so that this is not one of the exceptions here to be considered. In two other cases the crossed equalled the self-fertilised in height within five per cent; and in the fourth case the self-fertilised exceeded the crossed by above this limit. We have seen in Table 7/C that the whole advantage of a cross by a fresh stock is confined to fertility, and so it was with the intercrossed plants of the same stock compared with the self-fertilised, for the former were in fertility to the latter as 100 to 89. The intercrossed plants thus have at least one important advantage over the self-fertilised. Moreover, the flowers on the parent-plants when fertilised with pollen from another individual of the same stock yield far more seeds than when self-fertilised; the flowers in this latter case being often quite sterile. We may therefore conclude that a cross does some good, though it does not give to the crossed seedlings increased powers of growth.

7. Viscaria oculata.

The average height of the fifteen intercrossed plants to that of the fifteen self-fertilised plants was only as 100 to 97; but the former produced many more capsules than the latter, in the ratio of 100 to 77. Moreover, the flowers on the parent-plants which were crossed and self-fertilised, yielded seeds on one occasion in the proportion of 100 to 38, and on a second occasion in the proportion of 100 to 58. So that there can be no doubt about the beneficial effects of a cross, although the mean height of the crossed plants was only three per cent above that of the self-fertilised plants.

8. Specularia speculum.

Only the four tallest of the crossed and the four tallest of the self-fertilised plants, growing in four pots, were measured; and the former were to the latter in height as 100 to 98. In all four pots a crossed plant flowered before any one of the self-fertilised plants, and this is usually a safe indication of some real superiority in the crossed plants. The flowers on the parent-plants which were crossed with pollen from another plant yielded seeds compared with the self-fertilised flowers in the ratio of 100 to 72. We may therefore draw the same conclusion as in the last case with respect to a cross being decidedly beneficial.

9. Borago officinalis.

Only four crossed and four self-fertilised plants were raised and measured, and the former were to the latter in height as 100 to 102. So small a number of measurements ought never to be trusted; and in the present instance the advantage of the self-fertilised over the crossed plants depended almost entirely on one of the self-fertilised plants having grown to an unusual height. All four crossed plants flowered before their self-fertilised opponents. The cross-fertilised flowers on the parent-plants in comparison with the self-fertilised flowers yielded seeds in the proportion of 100 to 60. So that here again we may draw the same conclusion as in the two last cases.

10. Passiflora gracilis.

Only two crossed and two self-fertilised plants were raised; and the former were to the latter in height as 100 to 104. On the other hand, fruits from the cross-fertilised flowers on the parent-plants contained seeds in number, compared with those from the self-fertilised flowers, in the proportion of 100 to 85.

11. Phaseolus multiflorus.

The five crossed plants were to the five self-fertilised in height as 100 to 96. Although the crossed plants were thus only four per cent taller than the self-fertilised, they flowered in both pots before them. It is therefore probable that they had some real advantage over the self-fertilised plants.

12. Adonis aestivalis.

The four crossed plants were almost exactly equal in height to the four self-fertilised plants, but as so few plants were measured, and as these were all "miserably unhealthy," nothing can be inferred with safety with respect to their relative heights.

13. Bartonia aurea.

The eight crossed plants were to the eight self-fertilised in height as 100 to 107. This number of plants, considering the care with which they were raised and compared, ought to have given a trustworthy result. But from some unknown cause they grew very unequally, and they became so unhealthy that only three of the crossed and three of the self-fertilised plants set any seeds, and these few in number. Under these circumstances the mean height of neither lot can be trusted, and the experiment is valueless. The cross-fertilised flowers on the parent-plants yielded rather more seeds than the self-fertilised flowers.

14. Thunbergia alata.

The six crossed plants were to the six self-fertilised in height as 100 to 108. Here the self-fertilised plants seem to have a decided advantage; but both lots grew unequally, some of the plants in both being more than twice as tall as others. The parent-plants also were in an odd semi-sterile condition. Under these circumstances the superiority of the self-fertilised plants cannot be fully trusted.

15. Nolana prostrata.

The five crossed plants were to the five self-fertilised in height as 100 to 105; so that the latter seem here to have a small but decided advantage. On the other hand, the flowers on the parent-plants which were cross-fertilised produced very many more capsules than the self-fertilised flowers, in the ratio of 100 to 21; and the seeds which the former contained were heavier than an equal number from the self-fertilised capsules in the ratio of 100 to 82.

16. Hibiscus africanus.

Only four pairs were raised, and the crossed were to the self-fertilised in height as 100 to 109. Excepting that too few plants were measured, I know of nothing else to cause distrust in the result. The cross-fertilised flowers on the parent-plants were, on the other hand, rather more productive than the self-fertilised flowers.

17. Apium petroselinum.

A few plants (number not recorded) derived from flowers believed to have been crossed by insects and a few self-fertilised plants were grown on the opposite sides of four pots. They attained to a nearly equal height, the crossed having a very slight advantage.

18. Vandellia nummularifolia.

Twenty crossed plants raised from the seeds of perfect flowers were to twenty self-fertilised plants, likewise raised from the seeds of perfect flowers, in height as 100 to 99. The experiment was repeated, with the sole difference that the plants were allowed to grow more crowded; and now the twenty-four tallest of the crossed plants were to the twenty-four tallest self-fertilised plants in height as 100 to 94, and in weight as 100 to 97. Moreover, a larger number of the crossed than of the self-fertilised plants grew to a moderate height. The above-mentioned twenty crossed plants were also grown in competition with twenty self-fertilised plants raised from the closed or cleistogene flowers, and their heights were as 100 to 94. Therefore had it not been for the first trial, in which the crossed plants were to the self-fertilised in height only as 100 to 99, this species might have been classed with those in which the crossed plants exceed the self-fertilised by above five per cent. On the other hand, the crossed plants in the second trial bore fewer capsules; and these contained fewer seeds, than did the self-fertilised plants, all the capsules having been produced by cleistogene flowers. The whole case therefore must be left doubtful.

19. Pisum sativum (common pea).

Four-plants derived from a cross between individuals of the same variety were in height to four self-fertilised plants belonging to the same variety as 100 to 115. Although this cross did no good, we have seen under Table 7/C that a cross between distinct varieties adds greatly to the height and vigour of the offspring; and it was there explained that the fact of a cross between the individuals of the same variety not being beneficial, is almost certainly due to their having been self-fertilised for many generations, and in each generation grown under nearly similar conditions.

20, 21, 22. Canna warscewiczi.

Plants belonging to three generations were observed, and in all of three the crossed were approximately equal to the self-fertilised; the average height of the thirty-four crossed plants being to that of the same number of self-fertilised plants as 100 to 101. Therefore the crossed plants had no advantage over the self-fertilised; and it is probable that the same explanation here holds good as in the case of Pisum sativum; for the flowers of this Canna are perfectly self-fertile, and were never seen to be visited by insects in the hothouse, so as to be crossed by them. This plant, moreover, has been cultivated under glass for several generations in pots, and therefore under nearly uniform conditions. The capsules produced by the cross-fertilised flowers on the above thirty-four crossed plants contained more seeds than did the capsules produced by the self-fertilised flowers on the self-fertilised plants, in the proportion of 100 to 85; so that in this respect crossing was beneficial.

23. Primula sinensis.

The offspring of plants, some of which were legitimately and others illegitimately fertilised with pollen from a distinct plant, were almost exactly of the same height as the offspring of self-fertilised plants; but the former with rare exceptions flowered before the latter. I have shown in my paper on dimorphic plants that this species is commonly raised in England from self-fertilised seed, and the plants from having been cultivated in pots have been subjected to nearly uniform conditions. Moreover, many of them are now varying and changing their character, so as to become in a greater or less degree equal-styled, and in consequence highly self-fertile. Therefore I believe that the cause of the crossed plants not exceeding in height the self-fertilised is the same as in the two previous cases of Pisum sativum and Canna.

24, 25, 26. Nicotiana tabacum.

Four sets of measurements were made; in one, the self-fertilised plants greatly exceeded in height the crossed, in two others they were approximately equal to the crossed, and in the fourth were beaten by them; but this latter case does not here concern us. The individual plants differ in constitution, so that the descendants of some profit by their parents having been intercrossed, whilst others do not. Taking all three generations together, the twenty-seven crossed plants were in height to the twenty-seven self-fertilised plants as 100 to 96. This excess of height in the crossed plants, is so small compared with that displayed by the offspring from the same mother-plants when crossed by a slightly different variety, that we may suspect (as explained under Table 7/C) that most of the individuals belonging to the variety which served as the mother-plants in my experiments, had acquired a nearly similar constitution, so as not to profit by being mutually intercrossed.]

Reviewing these twenty-six cases, in which the crossed plants either do not exceed the self-fertilised by above five per cent in height, or are inferior to them, we may conclude that much the greater number of the cases do not form real exceptions to the rule,--that a cross between two plants, unless these have been self-fertilised and exposed to nearly the same conditions for many generations, gives a great advantage of some kind to the offspring. Of the twenty-six cases, at least two, namely, those of Adonis and Bartonia, may be wholly excluded, as the trials were worthless from the extreme unhealthiness of the plants. Inn twelve other cases (three trials with Eschscholtzia here included) the crossed plants either were superior in height to the self-fertilised in all the other generations excepting the one in question, or they showed their superiority in some different manner, as in weight, fertility, or in flowering first; or again, the cross-fertilised flowers on the mother-plant were much more productive of seed than the self-fertilised.

Deducting these fourteen cases, there remain twelve in which the crossed plants show no well-marked advantage over the self-fertilised. On the other hand, we have seen that there are fifty-seven cases in which the crossed plants exceed the self-fertilised in height by at least five per cent, and generally in a much higher degree. But even in the twelve cases just referred to, the want of any advantage on the crossed side is far from certain: with Thunbergia the parent-plants were in an odd semi-sterile condition, and the offspring grew very unequally; with Hibiscus and Apium much too few plants were raised for the measurements to be trusted, and the cross-fertilised flowers of Hibiscus produced rather more seed than did the self-fertilised; with Vandellia the crossed plants were a little taller and heavier than the self-fertilised, but as they were less fertile the case must be left doubtful. Lastly, with Pisum, Primula, the three generations of Canna, and the three of Nicotiana (which together complete the twelve cases), a cross between two plants certainly did no good or very little good to the offspring; but we have reason to believe that this is the result of these plants having been self-fertilised and cultivated under nearly uniform conditions for several generations. The same result followed with the experimental plants of Ipomoea and Mimulus, and to a certain extent with some other species, which had been intentionally treated by me in this manner; yet we know that these species in their normal condition profit greatly by being intercrossed. There is, therefore, not a single case in Table 7/A which affords decisive evidence against the rule that a cross between plants, the progenitors of which have been subjected to somewhat diversified conditions, is beneficial to the offspring. This is a surprising conclusion, for from the analogy of domesticated animals it could not have been anticipated, that the good effects of crossing or the evil effects of self-fertilisation would have been perceptible until the plants had been thus treated for several generations.

The results given in Table 7/A may be looked at under another point of view. Hitherto each generation has been considered as a separate case, of which there are eighty-three; and this no doubt is the more correct method of comparing the crossed and self-fertilised plants.

But in those cases in which plants of the same species were observed during several generations, a general average of their heights in all the generations together may be made; and such averages are given in Table 7/A; for instance, under Ipomoea the general average for the plants of all ten generations is as 100 for the crossed, to 77 for the self-fertilised plants. This having been done in each case in which more than one generation was raised, it is easy to calculate the average of the average heights of the crossed and self-fertilised plants of all the species included in Table 7/A. It should however be observed that as only a few plants of some species, whilst a considerable number of others, were measured, the value of the mean or average heights of the several species is very different. Subject to this source of error, it may be worth while to give the mean of the mean heights of the fifty-four species in Table 7/A; and the result is, calling the mean of the mean heights of the crossed plants 100, that of the self-fertilised plants is 87. But it is a better plan to divide the fifty-four species into three groups, as was done with the previously given eighty-three cases. The first group consists of species of which the mean heights of the self-fertilised plants are within five per cent of 100; so that the crossed and self-fertilised plants are approximately equal; and of such species there are twelve about which nothing need be said, the mean of the mean heights of the self-fertilised being of course very nearly 100, or exactly 99.58. The second group consists of the species, thirty-seven in number, of which the mean heights of the crossed plants exceed that of the self-fertilised plants by more than five per cent; and the mean of their mean heights is to that of the self-fertilised plants as 100 to 78. The third group consists of the species, only five in number, of which the mean heights of the self-fertilised plants exceed that of the crossed by more than five per cent; and here the mean of the mean heights of the crossed plants is to that of the self-fertilised as 100 to 109. Therefore if we exclude the species which are approximately equal, there are thirty-seven species in which the mean of the mean heights of the crossed plants exceeds that of the self-fertilised by twenty-two per cent; whereas there are only five species in which the mean of the mean heights of the self-fertilised plants exceeds that of the crossed, and this only by nine per cent.

The truth of the conclusion--that the good effects of a cross depend on the plants having been subjected to different conditions or to their belonging to different varieties, in both of which cases they would almost certainly differ somewhat in constitution--is supported by a comparison of the Tables 7/A and 7/C. The latter table gives the results of crossing plants with a fresh stock or with a distinct variety; and the superiority of the crossed offspring over the self-fertilised is here much more general and much more strongly marked than in Table 7/A, in which plants of the same stock were crossed. We have just seen that the mean of the mean heights of the crossed plants of the whole fifty-four species in Table 7/A is to that of the self-fertilised plants as 100 to 87; whereas the mean of the mean heights of the plants crossed by a fresh stock is to that of the self-fertilised in Table 7/C as 100 to 74. So that the crossed plants beat the self-fertilised plants by thirteen per cent in Table 7/A, and by twenty-six per cent, or double as much, in Table 7/C, which includes the results of the cross by a fresh stock.

TABLE 7/B.

A few words must be added on the weights of the crossed plants of the same stock, in comparison with the self-fertilised. Eleven cases are given in Table 7/B, relating to eight species. The number of plants which were weighed is shown in the two left columns, and their relative weights in the right column, that of the crossed plants being taken as 100. A few other cases have already been recorded in Table 7/C in reference to plants crossed by a fresh stock. I regret that more trials of this kind were not made, as the evidence of the superiority of the crossed over the self-fertilised plants is thus shown in a more conclusive manner than by their relative heights. But this plan was not thought of until a rather late period, and there were difficulties either way, as the seeds had to be collected when ripe, by which time the plants had often begun to wither. In only one out of the eleven cases in Table 7/B, that of Eschscholtzia, do the self-fertilised plants exceed the crossed in weight; and we have already seen they are likewise superior to them in height, though inferior in fertility, the whole advantage of a cross being here confined to the reproductive system. With Vandellia the crossed plants were a little heavier, as they were also a little taller than the self-fertilised; but as a greater number of more productive capsules were produced by the cleistogene flowers on the self-fertilised plants than by those on the crossed plants, the case must be left, as remarked under Table 7/A, altogether doubtful. The crossed and self-fertilised offspring from a partially self-sterile plant of Reseda odorata were almost equal in weight, though not in height. In the remaining eight cases, the crossed plants show a wonderful superiority over the self-fertilised, being more than double their weight, except in one case, and here the ratio is as high as 100 to 67. The results thus deduced from the weights of the plants confirm in a striking manner the former evidence of the beneficial effects of a cross between two plants of the same stock; and in the few cases in which plants derived from a cross with a fresh stock were weighed, the results are similar or even more striking.


Charles Darwin

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