40 Evolution and Adaptation 
form, in fact, would be, ex hypothese, 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 be spoken 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 
