Animals.—In animals the process is probably similar. Many animals, such as oysters, polyps, etc., are bisexual and self-fertilizing. But even in these, cross-fertilization must be very common, if not the rule. These animals usually live together in great numbers; the sperm-particles are extremely light and abundant. These are therefore carried by waves and currents, so that the waters are full of them, and a promiscuous cross-fertilization is unavoidable. In fact, there can be no doubt that it is in order to insure this cross-fertilization that the sperm-particles are so light and abundant; and the final cause of this, again, is that cross-fertilization produces better results than self-fertilization. But if so, then Nature will take steps not only to insure cross-fertilization, but to prevent self-fertilization. This in animals as in plants can only be done in two ways, viz., either by so placing the two organs that self-fertilization is impossible, or else by separating them in different individuals. A curious example of the former method is found in snails. These animals are bisexuals—i. e., have both ovary and spermary perfect, but these are so placed that self-fertilization is impossible. They, therefore, mutually cross-fertilize. The latter method, of course, produces unisexuality, so universal in higher animals; but the process was probably the abortion, in habitually cross-fertilizing bisexuals, of one organ or the other in different individuals until unisexuality is established.
If, then, we compare plants and animals, we find the steps similar in the two kingdoms. Bisexual animals living together in numbers, and cross-fertilized by waves and currents, correspond to anemophilous flowers cross-fertilized by winds. Mutually fertilizing bisexuals like snails correspond to orchidaceous plants, except that the cross which is voluntary in the former is effected by insects in the latter. Finally, unisexuals in animals correspond to Diœcia in plants. In both kingdoms unisexuality is derived from bisexuality[1]—in both because thus self-fertilization is prevented and cross-fertilization secured; and this, again, because thus a better result is secured in the offspring.
But the question has probably dwelt in your minds, "Why is it that cross-fertilization produces better results, i. e., stronger progeny, than self-fertilization?" There are probably two reasons: 1. The elaboration of both ovules and sperm in the same individual is wasteful of vital energy. The concentration of vital energy on one reproductive element secures that one product in a higher degree of perfection. Thus better sperm and better ovules combine to produce better ova and a stronger embryo. This is in accordance with the effect of differentiation of functions and organs of all kinds. 2. Again, in all cross-fertilization different individual characteristics are inherited by
- ↑ This must be taken as a general statement only. It is probable that in many cases the opposite or retrograde change occurred, and that the difficulties in the way of cross-fertilization compelled a return to self-fertilizing bisexuality. Such retrograde changes are common in evolution.
