on plants, guided, to a certain extent, by the experience of breeders of animals, I became convinced many years ago that it is a general law of nature that flowers are adapted to be crossed, at least occasionally, by pollen from a distinct plant." It was a direct deduction from his theory of natural selection that, since they are adapted for cross-fertilization, crossfertilization must be advantageous to them. Hence it was perfectly natural that he should like to verify it. "It often occurred to me," he said, "that it would be advisable to try whether seedlings from cross-fertilized flowers were in any way superior to those from selffertilized flowers. But as no instance was known with animals of any evil appearing in a single generation from the closest possible interbreeding, that is, between brothers and sisters, I thought that the same rule would hold good with plants, and that it would be necessary, at the sacrifice of too much time, to self-fertilize and intercross plants during several successive generations, in order to arrive at any results. I ought to have reflected that such elaborate provisions favoring cross-fertilization as we see in innumerable plants would not have been acquired for the sake of a distant and slight advantage, or of avoiding a

distant and slight evil. Moreover, the fertilization of a flower by its own pollen corresponds to a closer form of interbreeding than is possible with ordinary bisexual animals; so that an earlier result might have been expected."

He had carried the deduction far enough to warrant an effort to verify it, but was deterred by analogical reasoning from pursuing the matter further. Had he clung to his general theory and the special facts to be explained under it, he would, as he said himself, have reached an early result. The analogy, if it had served a good purpose, ought to have led him to reason that since continuous interbreeding is harmful among animals, although there are no special adaptations to prevent it, or to encourage the opposite, then, surely, the harmful effects of close breeding and the benefits of cross-fertilization ought to be very marked in plants, with their striking adaptation for crossfertilization. The analogy had clearly led him astray; and he was finally brought back to the subject by a different route.

"I was at last led to make the experiments recorded in the present volume from the following circumstance. For the sake of determining certain points with respect to inheritance, and without any thought of the effects of close.

interbreeding, I raised, close together, two large beds of self-fertilized and crossed seedlings from the same plant of Linaria vulgaris (common Toad Flax). To my surprise, the crossed plants, when fully grown, were plainly taller and more vigorous than the self-fertilized

ones.

"Bees incessantly visit the flowers of this Linaria, and carry pollen from one to the other; and if the insects are excluded the flowers produce extremely few seeds, so that the wild plants from which my seedlings were raised must have been intercrossed during all previous generations. It seems therefore quite incredible that the difference between the two beds of seedlings could have been due to a single act of self-fertilization; and I attributed the result to the self-fertilized seeds not having been well ripened, improbable as it was that all should have been in this state, or to some other accidental and inexplicable cause.' During the next season he raised two beds of carnations (Dianthus caryophyllus) in the same way and for the same purpose; the preceding generations in this case also must have been continuously cross-fertilized; again "the selffertilized seedlings were plainly inferior in height and vigor to the crossed."

"My attention," he said, "was now thoroughly aroused, for I could hardly doubt that the difference between the two beds was due to the one being the offspring of crossed and the other of self-fertilized flowers." After the effects of cross- and self-fertilization had been thus thrust upon him, he proceeded to make the exhaustive examination that ran through many years, and finally filled a volume. He foresaw the meaning of the adaptations for cross-fertilization and the character of the results, but was deterred by a false analogy from making the observations to which a careful study of the facts by themselves would have infallibly led him; and was finally driven to the subject again by the empirical observation of the facts that he had anticipated by reasoning. It may seem strange that the very consequences which theory led him to expect had to be twice forced upon the attention of one who was so quick to seize Nature's suggestions, before he could be brought to investigate them. But the strangeness of such an intellectual phenomenon is all due to the afterthought. Even in the sciences that are most rigidly deductive it is a common thing for the investigator to stumble indirectly upon results which he might have foreseen or often did more or less perfectly foresee.

In a more complex case, analogy led to a conclusion which, although it could not be verified, possesses great importance in relation to one of the principal difficulties in the way of the general theory of natural selection. In the course of the investigation on "Different Forms of Flowers on Plants of the same Species," he noticed the striking parallelism between the phenomena of hybridism and those of the heterostyled plants which he was studying.1 When once the parallelism was established, the remarkable and puzzling facts of hybridism doubtless furnished a solid analogical basis from which to foresee and scrutinize the results of crossing the different forms of heterostyled plants.

Difficulty in uniting two forms and sterility of their offspring had been almost universally regarded as a test of specific distinctness. Darwin showed clearly that this belief, although very generally true, is by no means universally so; and his work on heterostyled plants showed that all the phenomena of hybridism were displayed among forms that certainly belonged to the same species. He triumphantly overthrew the doctrine

doctrine that

1 Different Forms of Flowers on Plants of the same Species, pp. 242, 243.