should unite with and cause the development of certain gemmules—we do not in the least know; but it is the common attribute of most organic beings in which the sexes are separate.
The distinction between transmission and development is likewise seen in all ordinary cases of Reversion; but before discussing this subject it may be advisable to say a few words on those characters which I have called latent, and which would not be classed under Reversion in its usual sense. Most, or perhaps all, the secondary characters, which appertain to one sex, lie dormant in the other sex; that is, gemmules capable of development into the secondary male sexual characters are included within the female; and conversely female characters in the male. Why in the female, when her ovaria become diseased or fail to act, certain masculine gemmules become developed, we do not clearly know, any more than why when a young bull is castrated his horns continue growing until they almost resemble those of a cow; or why, when a stag is castrated, the gemmules derived from the antlers of his progenitors quite fail to be developed. But in many cases, with variable organic beings, the mutual affinities of the cells and gemmules become modified, so that parts are transposed or multiplied; and it would appear that a slight change in the constitution of an animal, in connection with the state of the reproductive organs, leads to changed affinities in the tissues of various parts of the body. Thus, when male animals first arrive at puberty, and subsequently during each recurrent season, certain cells or parts acquire an affinity for certain gemmules, which become developed into the secondary masculine characters; but if the reproductive organs be destroyed, or even temporarily disturbed by changed conditions, these affinities are not excited. Nevertheless, the male, before he arrives at puberty, and during the season when the species does not breed, must include the proper gemmules in a latent state. The curious case formerly given of a Hen which assumed the masculine characters, not of her own breed but of a remote progenitor, illustrates the connexion between latent sexual characters and ordinary reversion. With those animals and plants which habitually produce several forms, as with certain butterflies described by Mr. Wallace, in which three female forms and
the male exist, or as with the trimorphic species of Lythrum and Oxalis, gemmules capable of reproducing several widely-different forms must be latent in each individual.
The same principle of the latency of certain characters, combined with the transposition of organs, may be applied to those singular cases of butterflies and other insects, in which exactly one half or one quarter of the body resembles the male, and the other half or three quarters the female; and when this occurs the opposite sides of the body, separated from each other by a distinct line, sometimes differ in the most conspicuous manner. Again, these same principles apply to the cases given in the thirteenth chapter, in which the right and left sides of the body differ to an extraordinary degree, as in the spiral winding of certain shells, and as in the genus Verruca among cirripedes; for in these cases it is known that either side indifferently may undergo the same remarkable change of development.
Reversion, in the ordinary sense of the word, comes into action so incessantly, that it evidently forms an essential part of the general law of inheritance. It occurs with beings, however propagated, whether by buds or seminal generation, and sometimes may even be observed in the same individual as it advances in age. The tendency to reversion is often induced by a change of conditions, and in the plainest manner by the act of crossing. Crossed forms are generally at first nearly intermediate in character between their two parents; but in the next generation the offspring generally revert to one or both of their grandparents, and occasionally to more remote ancestors. How can we account for these facts? Each organic unit in a hybrid must throw off, according to the doctrine of pangenesis, an abundance of hybridised gemmules, for crossed plants can be readily and largely propagated by buds; but by the same hypothesis there will likewise be present dormant gemmules derived from both pure parent-forms; and as these latter retain their normal condition, they would, it is probable, be enabled to multiply largely during the lifetime of each hybrid. Consequently the sexual elements of a hybrid will include both pure and hybridised gemmules; and when two hybrids pair, the combination of pure gemmules derived from the one hybrid with the pure gemmules of the same parts derived from the other would
necessarily lead to complete reversion of character; and it is, perhaps, not too bold a supposition that unmodified and undeteriorated gemmules of the same nature would be especially apt to combine. Pure gemmules in combination with hybridised gemmules would lead to partial reversion. And lastly, hybridised gemmules derived from both parent-hybrids would simply reproduce the original hybrid form.[[927]] All these cases and degrees of reversion incessantly occur.
It was shown in the fifteenth chapter that certain characters are antagonistic to each other or do not readily blend together; hence, when two animals with antagonistic characters are crossed, it might well happen that a sufficiency of gemmules in the male alone for the reproduction of his peculiar characters, and in the female alone for the reproduction of her peculiar characters, would not be present; and in this case dormant gemmules derived from some remote progenitor might easily gain the ascendency, and cause the reappearance of long-lost characters. For instance, when black and white pigeons, or black and white fowls, are crossed,—colours which do not readily blend,—blue plumage in the one case, evidently derived from the rock-pigeon, and red plumage in the other case, derived from the wild jungle-cock, occasionally reappear. With uncrossed breeds the same result would follow, under conditions which favoured the multiplication and development of certain dormant gemmules, as when animals become feral and revert to their pristine character. A certain number of gemmules being requisite for the development of each character, as is known to be the case from several spermatozoa or pollen-grains being necessary for fertilisation, and time favouring their multiplication, will together account for the curious cases, insisted on by Mr. Sedgwick, of certain diseases regularly appearing in alternate generations. This likewise holds good, more or less strictly, with other weakly inherited modifications. Hence, as I have heard it remarked, certain diseases appear actually to gain strength by the intermission of a generation. The transmission of dormant gemmules during many successive generations is hardly in itself more improbable, as
previously remarked, than the retention during many ages of rudimentary organs, or even only of a tendency to the production of a rudiment; but there is no reason to suppose that all dormant gemmules would be transmitted and propagated for ever. Excessively minute and numerous as they are believed to be, an infinite number derived, during a long course of modification and descent, from each cell of each progenitor, could not be supported or nourished by the organism. On the other hand, it does not seem improbable that certain gemmules, under favourable conditions, should be retained and go on multiplying for a longer period than others. Finally, on the views here given, we certainly gain some clear insight into the wonderful fact that the child may depart from the type of both its parents, and resemble its grandparents, or ancestors removed by many generations.
Conclusion.
The hypothesis of Pangenesis, as applied to the several great classes of facts just discussed, no doubt is extremely complex; but so assuredly are the facts. The assumptions, however, on which the hypothesis rests cannot be considered as complex in any extreme degree—namely, that all organic units, besides having the power, as is generally admitted, of growing by self-division, throw off free and minute atoms of their contents, that is gemmules. These multiply and aggregate themselves into buds and the sexual elements; their development depends on their union with other nascent cells or units; and they are capable of transmission in a dormant state to successive generations.