REPRODUCTION AND SEX 199 



passive. It is the part of the female to lay up the surplus 

 material that she manufactures in the formation of germs 

 provided with the cytoplasm which the zygote will require 

 and stocked with yolk to serve as food in the early stages 

 of development. Sometimes, as in the hairy, warm-blooded 

 animals known as mammals, she also nourishes the young 

 formed from the zygote. The part of the male is freely to 

 expend his substance in finding the female, while the germs 

 that he forms are poor in cytoplasm, but actively motile 

 to reach and enter the egg. In the highest animals his 

 activity is directed also to the preservation of the female, 

 for whom he fights and forages. It is in activity, not in 

 size or strength or beauty, that the true difference between 

 the sexes consists, though the female is perhaps generally 

 the larger and the male relatively the stronger of the two, and 

 the greater physiological vigour of the male often appears, as 

 in the peacock and the stag, in the production of ornaments 

 and weapons which are not found in the other sex. By 

 this dimorphism of the parents two things are accomplished. 

 Firstly, a division of labour of the same kind that exists 

 between the gametes is brought about between the indi- 

 viduals that form them, one ensuring the union of the 

 gametes, the other the nourishment of the zygote. 1 

 Secondly, it is ensured that cross-fertilisation (p. 126) shall 

 take place, since the germs of one parent, being all of the 

 same kind, are unable to fertilise one another. 



Why conjugation should usually be cross-fertilisation 



it is hard to say, but the fact remains that as 

 fertilisation. a general rule it takes place in this way, and 



that there are often elaborate precautions for 

 ensuring that it should do so. The union of gametes from 

 the same parent (which must, of course, be a herma- 

 phrodite) is known as self-fertilisation, and it has been 

 shown that in some cases this produces enfeebled zygotes, 

 most of which fail to develop. On the other hand there 

 are animals known in which self-fertilisation is the only 

 kind of conjugation which occurs. Again, conjugation 



1 In Vorticella, as we have seen (p. 142), a curious imitation of sex 

 is found. The parents are here both hermaphrodites, but they are of 

 two kinds, of which one is active and the other is passive, as in parents 

 which possess true sex. 



