240 THE EVOLUTION THEORY 



But many of these species do not fertilize themselves, but fertilize 

 each other mutually ; and this, too, carries a great advantao-e with 

 it, because in sedentary animals the sperms will fertilize twice as 

 many individuals, if each contains eggs, than if half were exclusively 

 male. It is thus to some extent an economy of sperms, but at the 

 same time also of ova, which is effected by hermaphroditism : the 

 result is that these valuable products are wasted as little as possible. 

 On this account we find that not only sedentary, but also sluggish, 

 slow-moving animals are equipped with male and female organs of 

 reproduction, as, for instance, all our terrestrial snails. They fertilize 

 each other mutually : when two meet it is always as males and 

 females, and notwithstanding the sluggishness of movement, it 

 is not likely to happen that a snail does not attain to reproduction 

 because it has not found a mate. The same is true of the earth- 

 worms, which are likewise not adapted for making long journeys 

 in search of the opposite sex; they, and the leeches also, function 

 as male and female simultaneously, while their nearest relatives, 

 the marine Chsetopods, are of separate sexes, which may be associated 

 with their much greater power of free movement in the water. 



In these cases self-fertilization is often absolutely excluded ; it 

 may be physically impossible, and hermaphroditism therefore secures 

 cross-fertilization in such cases just as effectively as if the sexes were 

 separate. Similarly, in mam^ hermaphrodite flowers, as we have 

 already seen, the pollen is so constituted and so placed within the 

 flower that it cannot of itself make its way to the stigma. In oysters, 

 for instance, the young animal is male, and liberates into the water 

 an enormous quantity of minute spermatozoa, and therewith fertilizes 

 the older individuals, functioning only as females, which have grown 

 upon the same bank. At a later stage of its development the oyster 

 which was male becomes female, and produces only ova. This state 

 of affairs, of which I shall shortly mention another case, has been 

 called temiioorary hermaphroditism. In this case not only is self- 

 fertilization excluded, but close inbreeding also, since it is always 

 a young generation functioning simultaneously as males that mingles 

 with an older generation which has become female. 



It is (juite otherwise with parasites which live singly within 

 the body of a host : for these it was indispensabl}^ necessary that 

 they should not only produce both kinds of germ-cells, but that they 

 should unite the two kinds in fertilization, and they therefore possess 

 the power of self-fertilization. Thus, in the urinary bladder of the 

 frog, there occurs a flat-worm {Polystomum integerriinum) which 

 possesses special organs for pairing with another individual, but 



