40 Evolution and Adaptation 



form, in fact, would be, ex hypotJiese, better suited to live 

 in a different environment, and consequently we should not 

 expect always to find its remains in the same place as that 

 occupied by the parent species. This possibility of migration 

 of new forms into a new locality makes the interpretation 

 of the geological record extremely hazardous. 



Nevertheless, if the evolution of the entire animal and 

 plant kingdoms had taken place within the period between 

 the first deposits of stratified rocks and the present time, we 

 might still have expected to find, despite the imperfections 

 of the record, sufficient evidence to show how the present 

 groups have arisen, and how they are related to one another. 

 But, unfortunately, at the period when the history of the 

 rocks begins, nearly all the large groups of animals were 

 in existence, and some of them, indeed, as the trilobites 

 and the brachiopods, appear to have reached the zenith of. 

 their development. 



On the other hand, the subdivisions of the group of verte- 

 brates have evolved during the period known to us. It is 

 true that the group was already formed when our knowledge 

 of it begins, but, from the fishes onwards, the history of the 

 vertebrates is recorded in the rocks. The highest group of 

 all, the mammals, has arisen within relatively modern times. 

 The correctness of the transmutation theory could be as well 

 established by a single group of geological remains as by the 

 entire animal kingdom. Let us, therefore, examine how far 

 the theory is substantiated by the paleontological record of the 

 vertebrates. We find that the earliest vertebrates were fishes, 

 and these were followed successively by the amphibians, 

 reptiles, birds, and mammals, one of the last species of all 

 to appear being man himself. There can be little doubt that 

 this series, with certain limitations to bespoken of in a moment, 

 represents a progressive series beginning with the simpler 

 forms and ending with the more complicated. Even did we 

 not know this geological sequence we would conclude, from 



