424 HYBRIDISM In this case we appear to have actual evidence of different stages of increasing sterility in transitu, and this even within the limits of the same natural species. And if even such evidence as this can be resisted, there still remains one very important fact, which directly affects the whole alleged distinction between the sterility of natural species and the fertility of domestic breeds. This fact is that plants belonging to several species of the genus Passi- flora have been amply proved, not only to be completely fertile with plants belonging to other species, but even to be as completely infertile with plants belonging to their own. Thus fruit could not be obtained from P. alata and P. racemosa except by reciprocally fertilizing them with each other s pollen ; and similar facts have been observed by several experimentalists with regard to four or five other species of this genus. The fullest details on the subject are those given by Mr Scott in the Journal of the Linnean Society, vol. viii. p. 168. Plants belonging to three species of the genus, viz., P. racemosa, coerulea, and alata, flowered for many years in Edinburgh, but, though repeatedly fertilized by Mr Scott and others with their own pollen, never produced seed. But when mutually crossed in various ways they all produced seed. After quoting this case Mr Darwin adds : " Returning to P. alata, I have received (1866) some interesting details from Mr Robinson Munro. Three plants, including one in England, have already been mentioned which were inveterately self-sterile, and Mr Munro informs me of several others which, after repeated trials during many year.s, have been found in the same predicament. At some other places, however, this species fruits realily when fertilized with its own pollen. At Taymouth Castle there is a plant which was formerly grafted by Mr Donaldson on a distinct species, name unknown, and ever since the operation it has produced fruit in abundance by its own pollen, so that this small and unnatural change in the state of this plant has restored its self- fertility. Some of the seedlings from the Taymouth Castle plant were found to be not only sterile with their own pollen, but with each other s pollen and with the pollen of distinct species. Pollen from the Taymouth plant failed to fertilize certain plants of the same species, but was successful on one plant in the Edinburgh Botanic Gardens. Seedlings were raised from this latter union, and some of their flowers were fertilized by Mr Munro with their own pollen ; but they were found to be as self-impotent as the mother plant had always proved, except when fertilized by the grafted Taymouth plant, and except, as we shall see, when ferti lized by her own seedlings. Yet Air Munro fertilized eighteen flowers on the self-impotent mother plant with pollen from these her own self-impotent seedlings, and obtained, remarkable as the fact is, eighteen fine capsules full of excellent seed. I have met with uo case in regard to plants which shows so well as this of P. alata on what small and mysterious causes complete fertility or complete sterility depends." These cases in the genus Passiflora, although so highly remarkable, are not wholly unique. There is not, indeed, any other case of a natural species, the members of which are only fertile with members of another species; but there are several cases of natural species, the members of which are self-impotent, though freely fertile either with other plants of the same species, or with plants of different though allied species. This may perhaps be regarded as a transitional stage between the ordinary condition of plants and the extraordinary condition that obtains among species of the genus Passiflora. It occurs in individual plants of certain species of Lobelia and Verbascum, and among several genera of orchids. The cases of the latter are particularly remarkable, inasmuch as Fritz Miiller found from numerous experiments, not only that individual plants belonging to the several species were not fertilized by their own pollen, while freely so by pollen taken from distinct species, and even from distinct genera, but that the plant s own pollen was positively deleterious to its stigma, and acted as a poison to the destruction of the flower. So much, then, for the facts which go to prove on what slight constitutional differences sterility may depend, and the consequent probability there is that it should generally be found to accompany a change of organization which is sufficiently great to be regarded by naturalists as a specific distinction. But the pleading must not end here. For there still remains to be adduced the fact, already mentioned as one of the general facts of hybridism, that " the degree of both kinds of fertility varies in the case of different species, and in that of their hybrid progeny, from absolute sterility up to complete fertility." As a matter of fact, the distinction between natural species and domestic varieties, upon which the whole discussion has hitherto proceeded, is in itself untenable ; the infertility of natural species when crossed, although without question the general rule, is nevertheless not the invariable rule. We need not point to the highly anomalous case of Passiflora recently mentioned in another connexion, and probably to be explained as the result of cultivation ; for we appear to have sufficient evidence without it. It is true that a great deal of negative evidence has been published upon this point by very competent experimentalists ; but it seems impossible to resist the positive evidence of the Hon. and Rev. W. Herbert, whose distinguished success in hybridizing Mr Darwin attributes to " great horticultural skill, and to his having hot-houses at his command." Of his many results, which from being of a positive kind can scarcely be suspected of inaccuracy, it will be enough to quote the following: "Every ovule in a pod of Crinum capense fertilized by C. revolutum produced a plant, which I never saw to occur in a case of its natural fecundation." Thus, as Mr Darwin in alluding to this case remarks, " we have perfect, or even more than commonly perfect, fertility in a first cross between two distinct species." So far, then, as one side of the question before us is concerned, or that relating to the mutual infertility of natural species, enough has been said to .show that it presents no real difficulty to the theory of descent. Indeed, in view of all that has now been said, the difficulty, as Mr Darwin has observed, is not so much to account for the sterility of natural species, as it is to account for the con tinued, or even increased, fertility of our domestic varieties. Turning, therefore, to this other side of the question, we have to remember that the very same sensitiveness of the reproductive system, which in some cases leads to infertility under a change in the conditions of life, in other cases leads to increased fertility under an apparently similar change in the conditions of life. Thus it is that domestication produces such apparently capricious results with regard to fertility inducing all degrees of infertility in some wild species, while not at all impairing, or even increasing, fertility in others. Consequently, when the question is as to why our domestic varieties do not become sterile inter se when so many natural species have become so, the answer is that the mere fact of their domestication proves that their wild or parent stocks must have been some of those species whose reproductive systems were not highly sensitive to changes in their condition of life, and therefore species which " might be expected to produce varieties little liable to have their reproductive systems injuriously affected by the act of crossing with other varieties which had originated in a like manner." Thus, on the inherently necessary view that our domestic varieties have all proceeded from species which were not easily affected in the direction of sterility, we are not surprised that under variation their reproductive systems should continue to manifest a high degree of tolerance. To this must be added that domestication, if it does not produce sterility, seems well calculated to increase fertility. For if the causes inducing sterility (whatever they may be) are absent, ample and regular nutrition, combined with innumerable lesser benefits attending domestication, may well be supposed to favour fertility.
And, as a matter of fact, according to Pallas, there is a
