CRUSTACEA AND MOLLUSCS 255 



their union the tiny female lays a single egg, which lasts 

 through the winter, and makes a beginning of a new 

 parthenogenetic generation in the following spring. 



In this case it is clear that the union of male and 

 female does not lead to multiplication. On the contrary, 

 there is only one egg, and so one individual, from each 

 pair. If the phylloxera depended on sexual propagation 

 alone, it would soon become extinct. Its propagation 

 and multiplication are exclusively parthenogenetic. 



But why do the males appear at all, we may ask, 

 when the phylloxera reproduces so much better 

 parthenogenetically ? And when we see that these 

 animals can dispense with sexual generation, why is 

 the same process not possible in other cases ? What 

 advantage is it to organisms that a new individual can 

 only be formed by the union of the generative products 

 of two sexes ? 



We call the ovum and the sperm by the common 

 name of "germ-cells." They are both the germs of 

 the new individual, and originally either the ovum, as 

 in our illustration, or the spermatozoon, could develop 

 gui^e independently into an animal. We shall see more 

 about this later on, but may point out here that the 

 spermatozoon and the ovum are structures of equal 

 value as regards heredity. The ovum contains the 

 bodily and mental characteristics of the mother and her 

 ancestors ; the spermatozoon the features of the father 

 and his ancestors. And just as a new individual can be 

 formed in the ovum from these characteristics by a 

 certain composition, as the above examples show, so 

 there is no essential reason why one should not be 



