^^^^ INTRODUCTION '^^''" 



All animals must eat. It is essential therefore that 

 we understand the origin of foods and just how the 

 necessary foodstuffs are continuously provided for the 

 plants and animals. We must also understand the 

 very varying conditions under which foodstuffs are 

 available and which therefore must be met by any 

 animal or plant making use of them. The very di- 

 verse ways of meeting these conditions and nature's 

 safeguards which prevent the increase of any form of 

 life to a point endangering its food supply have a most 

 important bearing on the origin and development of 

 animal forms. Especially important is an adequate 

 understanding of the differences in the conditions 

 which affect the animals living on the land and in 

 the sea. 



In addition to the animals living at the present time 

 we know many other kinds which flourished in past 

 ages, the remains of which have been preserved as fos- 

 sils in the rocks. As we go back further and further 

 ^ into the geologic past we find that animal life becomes 

 more and more different from the animal life we know 

 today. This fact is obvious and undeniable. Equally 

 obvious and undeniable are certain other facts which 

 hitherto have passed unnoticed. A rather detailed 

 survey of the fossils is therefore essential to a proper 

 understanding of the development of animal forms. 



Fossil or living, all animals are divided into differ- 

 ent kinds or species. The species is the unit by which 

 the animal world is measured and in terms of which it 

 is discussed. How may this unit be defined? Is it a 

 fixed and stable entity, or is it a repressed force kept 



bdi] 



