74 THE EVOLUTION OF SEX. 



It is e(|iially necessary to emphasise, 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 fertilise 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-fertilisation among herma- 

 phrodites has been explained in terms of the disadvantage of 

 the process. In reality, however, this is putting the cart before 

 the horse. In hermaphrodites, 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- 

 fertilisation on the health of the species, l)ut simply because the 

 simultaneous co-existence of opposite physiological processes is 

 in varying degrees prohibited. More technically, dichogamy is 

 not a subsequent result of the disadvantage of self-fertilisation, 

 but cross-fertilisation is the subsequent result of increasing 

 dichogamy. 



Self-fertilisation 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 tape-worms or cestodes ; 

 also in the curious thread-worm AngiostoinunK and probably 

 in ctenophores, and in some other cases. In regard to some 

 cases, e.g.^ 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-fertilisation 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 the pollen of a given 

 flower is quite ino})erative on the ovule of the same flower, or 

 has the result of producing weakly offspring. Then there are 

 a great variety of mechanical devices, as the result of which it 

 is more or less physically impossible for the pollen of the 



