196 PRIMITIVE FORMS OF LIFE 



ment of the genital organs, causing what Giard has called 

 parasitic castration. But this is not a matter of castration alone. 

 The appearance of the host may be modified to such an extent 

 that it appears to constitute a new species, as Jean Perez 

 showed in the case of the stylopized Andrena. These are all 

 specific instances of a general phenomenon which, when con- 

 sidered in its amplitude, ends by including within its own 

 circumference all the results of the struggle for existence and 

 natural selection. The present distribution and arrangement 

 of fauna and flora presupposes, in fact, a reciprocal adaptation 

 of organisms such that, without too much endangering each 

 other, they can live side by side. We shall have to apply this 

 principle in the course of our chapters on life during the various 

 geological periods. 



Once these various organic types had been fully and securely 

 established, sea and land became rapidly populated. The 

 struggle for existence became more and more bitter, and if it 

 created nothing new, it did at least determine what could live 

 and what must die, assure the conservation and the 

 development of the most vital forms, and cause those gaps 

 among living organisms that mark off one species from the 

 other. This is what we shall find did actually happen when we 

 come to study the great geological periods. 



