^'^ INTRODUCTION ^^'^ 



within bounds by rigid limitations which it is unable 

 to escape? 



But the discussion of a species is quite inadequate 

 and meaningless without a survey of the life histories 

 of animals and a mention of the varied and complex 

 forms more or less widely different from the adult 

 form which most animals assume at one time or an- 

 other in the passage from the egg to the final stage. 

 And in addition we must review the varied processes 

 by which the continuity of life from one generation 

 to the next and from one individual to another is 

 assured and the significance and relative importance 

 of these processes. 



Such is the background with which any tenable 

 theory of the development and diversification of 

 animal forms must harmonize. The idea that the 

 earliest forms of life were feeble helpless things exist- 

 ing in an ideal world especially adapted to them has 

 nothing to support it. So far as we can learn the 

 conditions on the earth since life began have always 

 been essentially the same — varying widely in climatic 

 and other details from age to age and from place to 

 place but always in their broader features essentially 

 the same. 



No matter what they are, all animals arise from a 

 single cell, and the body of every animal is composed 

 of one or many cells which are always similar in 

 structure. All animals must therefore be interpreted 

 in terms of a single cell. Furthermore, all living 

 things arise only as the children of other living things. 



So the problem is to construct a figure which, begin- 



[xiii] 



