The outcome of these different crossings are most curious. In the table, page 364, the results of the four combinations are given. It will be seen at once that the legitimate unions give more capsules, and the seeds weigh more, than in the illegitimate unions.
The behavior of the offspring from seeds of legitimate and illegitimate origin is even more astonishing. Darwin found in Primula veris (the form just described) that the seeds from the short-styled form fertilized with pollen from the same form germinated so badly that he obtained only 14 plants, of which 9 were short-styled and 5 long-styled. The long-styled form fertilized with its own-styled pollen produced “in the first generation 3 long-styled plants. From their seed 53 long-styled grandchildren were produced; from their seed 4 long-styled great-grandchildren; from their seed 20 long-styled great-great-grandchildren; and lastly, from their seed 8 long-styled and 2 short-styled great-great-great-grandchildren.”
Nature of Union | Number of Flowers Fertilized | Number of Seed Capsules | Maximum of Seeds in any one Capsule | Minimum of Seeds in any one Capsule | Average No. of Seeds per Capsule |
|---|---|---|---|---|---|
| Long-styled form by pollen of short-styled form: Legitimate union. | 10 | 6 | 62 | 34 | 46.5 |
| Long-styled form by own-form pollen: Illegitimate union. | 20 | 4 | 49 | 2 | 27.7 |
| Short-styled form by pollen of long-styled form: Legitimate union. | 10 | 8 | 61 | 37 | 47.7 |
| Short-styled form by own-form pollen: Illegitimate union. | 17 | 3 | 19 | 6 | 12.1 |
| The two legitimate unions together. | 20 | 14 | 62 | 37 | 47.1 |
| The two illegitimate unions together. | 30 | 7 | 49 | 2 | 35.5 |
From other long-styled plants, fertilized with their own-form pollen, 72 plants were raised, which were made up of 68 long-styled and 4 short-styled. In all, 162 illegitimate unions of this sort produced 156 long-styled and 6 short-styled plants. It is evident from these results that the long-form pistils, fertilized with pollen from flowers of the same pistil-form (from other individuals as a rule), tend to produce the same form as their parents, although occasionally the other form. The fertility of these plants from an illegitimate union is found to be very low. Darwin observed that sometimes the male and female organs of these plants were in a very deteriorated condition. It is interesting to notice, in this connection, that in another species, Primula sinensis, illegitimate plants from long-styled parents were vigorous, but the flowers were small and more like the wild form. They were, however, perfectly fertile.
Illegitimate plants from short-styled parents were dwarfed in stature, and often had a weakly constitution. They were not very fertile inter se, and remarkably infertile when legitimately fertilized. This kind of result, where a difference in the power of mutual intercrossing exists between two forms, recalls in many ways the difference in the results of crossing of different species of animals and plants, especially those cases in which a cross can be made in one way more successfully than in the other.
The heterostyled trimorphic plants, of which Lythrum salicaria, Figure [5 C, D, E], may be taken as an example, are even more remarkable. There are three different kinds of flowers: in one the pistil is long and there is a medium and a short set of stamens; in another the pistil is of intermediate length and there is a long set and a short set of stamens; in the third kind the pistil is short, and there is a medium and a long set of stamens. There are possible only six sorts of legitimate unions between these three sets of flowers. No less than twelve kinds of illegitimate unions may occur. In regard to the difference in the sizes of the pollen grains, those from the long-styled form are the largest, from the mid-styled form next, and from the short-styled form the smallest. The extreme difference is as 100 to 60. “Nothing shows more clearly the extraordinary complexity of the reproductive system of this plant than the necessity of making eighteen distinct unions in order to ascertain the relative fertilizing power of the three forms.” Darwin tried the effect of each of these combinations, making 223 unions in all. The results are surprising. Comparing the outcome of the six legitimate unions with the twelve illegitimate ones, the following results were obtained:—
Nature of Union | Number of Flowers Fertilized | Number of Capsules Produced | Average No. of Seeds per Capsule | Average No. of Seeds per Flower Fertilized |
|---|---|---|---|---|
| The 6 legitimate unions | 75 | 56 | 96.29 | 71.89 |
| The 12 illegitimate unions | 146 | 36 | 44.72 | 11.03 |
This table shows that the fertility of the legitimate to that of the illegitimate is as 100 to 33, as judged by the flowers that produced capsules; and as 100 to 46 as judged by the average number of seeds per capsule. It is evident, therefore, that “it is only the pollen from the longest stamens that can fully fertilize the longest pistil; only that from the mid-length stamens, the mid-length pistil; and only that from the shortest stamens, the shortest pistil.”
Darwin tries to connect this fact with the visits of insects to the flowers. He says: “And now we can comprehend the meaning of the almost exact correspondence in length between the pistil in each form and of a set of six stamens in two of the other forms; for the stigma of each form is thus rubbed against that part of the insect’s body which becomes charged with the proper pollen.” A further conclusion that Darwin draws is “that the greater the inequality in length between the pistil and the set of stamens, the pollen of which is employed for its fertilization, by so much is the sterility the more increased.” Darwin also makes the following significant comment on the problem here involved: “The correspondence in length between the pistil in each form, and a set of stamens in the other two forms, is probably the direct result of adaptation, as it is of the highest service to the species by leading to full and legitimate fertilization.” He points out, on the other hand, that the increased sterility of the illegitimate unions, in proportion to the inequality in length between the pistil and the stamens employed, can be of no service at all. Neither can this relation have any connection with the facility for self-fertilization. “We are led, therefore, to conclude that the rule of increased sterility in accordance with increased inequality in length between the pistils and stamens is a purposeless result, incidental on those changes through which the species has passed in acquiring certain characters fitted to insure the legitimate fertilization of the three flowers.”
In regard to the plants that were raised from the seeds from legitimate and illegitimate unions, Darwin found in Lythrum that of twelve illegitimate unions two were completely barren, and nearly all showed lessened fertility; only one approached complete fertility. Darwin lays much emphasis on the close resemblance in the sterility of the illegitimate unions, and the sterility of different species when crossed. In both cases every degree of sterility is met with, “from very slightly lessened fertility to absolute barrenness.” The importance of this comparison cannot, I think, be overestimated, for, if admitted, it indicates clearly that the infertility between species cannot be used as a criterion of their distinctness, because here, in individuals belonging to the same species, we find sterility between pistils and stamens of different lengths. If, as I shall urge below, we must consider these different forms of Primula the results of a mutation, and not the outcome of selection as Darwin supposed, then this relation in regard to infertility becomes a point of great interest.