70 THE EVOLUTION OF SEX. 



ova and sperms produced within the compass of one small organ, but 

 each little corner of the organ shows female cells forming on the walls 

 and male cells in the center. It has been justly suggested by Platner 

 that the outer cells are the better nourished: they therefore naturally 

 become developed into anabolic ova. 



VII. Self-fertilization. — We have noted above that though 

 male and female organs be present in the same organism, they tend 

 to become mature at different times, and that the more the closer the 

 seats of formation of the two kinds of elements. It is equally neces- 

 sary to emphasize that though both male and female elements may be 

 produced in the same plant or animal, it is probably exceptional for 

 the ovule to be penetrated by a pollen-cell from the same flower, and 

 it is certainly rare for an animal to fertilize its own ova. 



It is believed by breeders of higher animals that " close-breeding" 

 beyond a certain point is dangerous to the welfare of the breed. The 

 offspring tend to be abnormal or unhealthy. In view of this, the 

 rarity of self-fertilization among hermaphrodites has been explained 

 in terms of the disadvantage of the process. In reality, however, this 

 T 's putting the cart before the horse. In hermaphrodities, we take it 

 that the two kinds of sexual elements mature and are liberated at 

 different times, not because of any reaction of the disadvantageousness 

 of self-fertilization on the health of the species, but simply because 

 the simultaneous coexistence of opposite physiological processes is in 

 varying degrees prohibited. More technically, dichogamy is not a 

 subsequent result of the disadvantage of self-fertilization, but cross- 

 fertilization is the subsequent result of increasing dichogamy. 



Self-fertilization does, however, occur as an exception among 

 animals, — thus, in all probability, in the exceptional fish Serranus; 

 certainly in many parasitic flukes or trematodes ; ' ' commonly, if not 

 universally," in tapeworms or cestodes; also in the curious thread- 

 worm Angiostomum, and probably in ctenophores, and in some other 

 cases. In regard to some cases — for example, among hermaphrodite 

 bivalves (where the sperms are usually wafted in with the water) — it 

 is impossible as yet to say whether self-impregnation does or does not 

 occur. Some curious, but not very reliable, observations are on 

 record in regard to self-impregnation in casually hermaphrodite 

 insects. 



Arguing from the bad effects of close breeding among higher 

 animals, Darwin and others have called attention to the numerous 

 contrivances among plants which are said to render self-fertilization 

 impossible. It must again be said that this survival of a very old way 

 of explaining facts — in terms of their final advantage — is not really 

 a causal explanation at all. It has been pointed out that in some cases 



