I 



EVOLUTION OF LIFE IN RELATION TO CLIMATIC 

 AND GEOLOGICAL CHANGE 



1 . The need for generality in zoology 



The aim of any zoological study is to know about the life of the ani- 

 mals concerned. Our object in this book is, therefore, to help the 

 reader to learn as much as possible about all the vertebrate animal life 

 that has ever been. Thinking of the great numbers of types that have 

 existed since the first fishes swam in the Palaeozoic seas, one might 

 well be appalled by such a task: to describe all these populations in 

 detail would indeed demand a huge treatise. However, in a well- 

 developed science it should be possible to reduce the varied subject- 

 matter to order, to show that all differences can be understood to have 

 arisen by the influence of specified factors operating to modify an 

 original scheme. Animal and plant life is so varied that it has not yet 

 proved possible to systematize our knowledge of it as thoroughly as 

 we should wish. Thinking, again, of the variety of vertebrate lives, 

 it may seem impossible to imagine any general scheme and simple set 

 of factors that would include so many special circumstances. Yet 

 nothing less should be the aim of a true science of zoology. Too often 

 in the past we have been content to accumulate unrelated facts. It is 

 splendid to be aware of many details, but only by the synthesis of 

 these can we obtain either adequate means for handling so many 

 data or knowledge of the natures we are studying. In order to know 

 life — what it is, what it has been, and what it will be — we must 

 look beyond the details of individual lives and try to find rules govern- 

 ing all. Perhaps we may find the task less difficult than expected. 

 Even an elementary anatomical and physiological study shows that 

 all vertebrates are built upon a common plan and have certain simi- 

 larities of behaviour. Our object will be to come to know the nature 

 of this plan of life, of structure, and action, to show how it is modified 

 in special cases and how each special case is also an example of a 

 general type of modification. 



Since the problem arises from the variety of animals that have 

 lived and live today, our central task is obviously to inquire into the 

 reason for the existence of so much difference. If vertebrate life began 

 as one single fish-like type, why has it not continued as such until 

 now? Why, instead of numerous identical fishes, are there countless 



