“Most, or perhaps all, of 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.” This is seen in cases of castration or when the sexual organs from any cause have become functionless. The sex in which such changes are brought about tends to develop the secondary sexual characters of the other sex. The normal development of the secondary characters proper to the sex of the individual may be explained by a slight difference in the elective affinity of the cells so that they attract the corresponding gemmules rather than those of the opposite sex, which as we have seen are also present.
The male characters of the male sex are in many species latent except at certain seasons of the year, and in both sexes the proper characters are latent until sexual maturity. All such latent characters are closely connected with the cases of ordinary reversion. The appearance (whether seasonal or in the course of development) of cells with affinities for the latent gemmules explains the development of the characters in question.
Certain butterflies and plants (e.g. Lythrum) produce two or more separate forms of individuals. In these cases each individual includes the latent gemmules of the other forms as well as its own. Hermaphroditism in unisexual species, and especially in the occasional cases of insects in which the right side of the body is one sex and the left side the other, the line of separation dividing the individual into two equal halves, can be explained by slight abnormal changes in the affinities of cells for gemmules, so that a certain group of cells, or all the cells on one side of the body, attract the gemmules which would normally have remained latent.
Reversion is induced by a change of conditions and especially by crossing. The first results of crossing are usually intermediate between the parents, but in the next generation there is commonly reversion to one or both parent-forms, or even to a more remote ancestor. The existence of abundant hybridised gemmules is shown by the propagation of the cross in a true form by means of buds; but dormant gemmules from the parent-form are also present and multiply. In the sexual elements of the hybrid there are both pure and hybrid gemmules, and the addition of the pure gemmules in one sex to those in the other accounts for the reversion, especially if we assume that pure “gemmules of the same nature would be especially apt to combine.” Partial reversion on the one hand, and the reappearance of the hybrid form on the other, would be respectively due to a combination of pure with hybrid gemmules, and of the hybrid gemmules from both parent hybrids.
When characters which do not blend exist in the parents, crossing may result in an insufficiency of gemmules from the male alone and from the female alone, and then dormant ancestral gemmules might have the opportunity of development, and thus cause reversion. Similarly certain conditions might favour the increase and development of dormant gemmules. Diseases appearing in alternate generations, or gaining strength by the intermission of a generation, may be due to the increase of the gemmules in the intervening time, and the same explanation may hold for the sudden and irregular increase of a weakly inherited modification.
Darwin ends his general conclusions with these words:—
“No other attempt, as far as I am aware, has been made, imperfect as this confessedly is, to connect under one point of view these several grand classes of facts. An organic being is a microcosm—a little universe, formed of a host of self-propagating organisms, inconceivably minute and numerous as the stars in heaven.”
CHAPTER XXII.
PANGENESIS AND CONTINUITY OF THE GERM-PLASM: DARWIN’S CONFIDENCE IN PANGENESIS.
PANGENESIS.