INTRODUCTION 



37 



conversion of one species into another usually 

 requires a longer period than the whole of human 

 history, the 6,000 years of which are only an hour 

 in comparison with the immense duration of the history 

 of the earth. We saw above that it takes a long time 

 for grey hares to become white ; and we know from 

 geology that, as a matter of fact, all the terrestial 

 epochs comprise enormous periods of time. The latest 

 of them, the tertiary period, is calculated at several 

 hundred thousand years. Geology also gives us the 

 best idea of the mutability of species, as we find at 

 the bottom of the sea the remains of animals of 

 remote ages, which are totally different from the 

 animals of our time. Geologists can distribute the 

 various strata of the earth's crust in their chronological 

 succession. These strata are the pages of a book that 

 it has taken millions and millions of years to write. 

 Nowhere in it do we find the animals of our own 

 time, or at all events only in the very latest periods 

 of the earth's history j and the later the remains of 

 extinct animals are, from the geological point of view, 

 the closer do they come to the living fauna. It seems 

 clear, therefore, that the animals of modern times 

 were not present at the first creation of life, but only 

 came into existence at a later date, and succeeded 

 other animals in the dominion of the earth. But 

 whence did they come so suddenly, if they were not 

 evolved from other animals ? Every animal lies in 

 the body of another before birth, in the form of an 

 egg or ovum. But, clearly, before our animals 

 appeared there were only forms of a different character ; 



