Darwin’s idea that the small number of gemmules in the unfertilized egg may account for the lack of power of such eggs to develop until they are fertilized, has been shown to be incorrect by recent results in experimental embryology. We now know that many different kinds of stimuli have the power to start the development of the egg. Moreover, we also know that if a single spermatozoon is supplied with a piece of egg-protoplasm without a nucleus, it suffices to cause this piece of protoplasm to develop.
In the case of regeneration, which Darwin also tries to explain on the pangenesis hypothesis, we find that there is no need at all for an hypothesis of this sort; and there are a number of facts in connection with regeneration that are not in harmony with the hypothesis. For instance, when a part is cut off, the same part is regenerated; but under these circumstances it cannot be imagined that the part removed supplies the gemmules for the new part. Darwin tries to meet this objection by the assumption that every part of the body contains gemmules from every other part. But it has been shown that if a limb of the newt is completely extirpated, a new limb does not regenerate; and there is no reason why it should not do so on Darwin’s assumption that germs of the limb exist throughout the body.
The best-authenticated cases of the influence of the male on the tissues of the female are those in plants, where one species, or variety, is fertilized by another. Thus, if the orange is fertilized by the pollen of the lemon, the fruit may have the color and flavor of the lemon. Now the fruit is a product of the tissues of the ovary of the female, and not a part of the seedling that develops in the fruit from the cross-fertilized egg-cell. Analogous cases are recorded for the bean, whose pods may have their color influenced by fertilizing the flower with pollen of another variety having pods of a different color. In these cases we do not know whether the color of the fruit is influenced directly by the foreign pollen, or whether the influence is through the embryo that develops from the egg-cell. The action may appear to be the same, however, in either case; but because it seems probable here that there is some sort of influence of one tissue on another, let us not too readily conclude that this is brought about through any such imaginary bodies as gemmules. It may be directly caused, for instance, by some chemical substance produced in the young hybrid plant. If this is the case, the result would not be different in kind from that of certain flowers whose color may be influenced by certain chemical substances in the soil.