THE GENESIS OF SEX. 171 



Animals. — In animals the process is probably similar. Many ani- 

 mals, 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 car- 

 ried 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 oi'der 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-fertili- 

 zation. 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 sperraary perfect, but these are so placed that self-fertilization is 

 impossible. They, therefore, mutually cross-fertilize. The latter meth- 

 od, of coarse, produces unisexuality, so universal in higher animals ; 

 but the process was probably the abortion, in habitually cross-fertiliz- 

 ing 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 aneraophilous 

 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 Dicecia in plants. In both king- 

 doms unisexuality is derived from bisexuality * — 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 loaste- 

 fal of vital energy. The concentration of vital energy on one repro- 

 ductive element secures that one product in a higher degree of perfec- 

 tion. 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. 



