RETROSPECT AND PROSPECT 275 



Retrospect and Prospect 



The initial question raised in this volume arises as soon 

 as we undertake a summary of evolution as we see it in the 

 retrospect of the ages. 



Does the energy conception of evolution bring us nearer 

 to the causes either of the origin or of the transformation of 

 characters? Before answering these crucial questions let us 

 see what our brief survey has taught us as to the kind of causes 

 to look for. 



The foregoing comparison in the second part of this vol- 

 ume of the evolutionary development that has taken place 

 in many series of animals belonging to the five great classes 

 of vertebrates — fishes, reptiles, amphibians, birds, and mam- 

 mals — in response to twelve different kinds of environment, 

 gives repeated evidence of their continuous powers of ever- 

 plastic adaptation, not only to one kind of physical and 

 life environment, but to any direct, reversed, or alternating 

 change of environment which a group of animals may en- 

 counter either on its own initiative or by force of circum- 

 stances. 



In the large vertebrates we are enabled to observe and 

 often to follow in minute details this continuous adaptation 

 not merely in one, but in hundreds and sometimes in thou- 

 sands of characters. In this respect a vertebrate differs from 

 a relatively simple plant organism like the pea or the bean 

 on which some of the prevailing conceptions of evolution have 

 been grounded. In the well-ordered evolution of these single 

 characters we have a picture like that of a vast army of sol- 

 diers; the organism as a whole is Hke the army; the "char- 

 acters" are like the individual soldiers; and the evolution of 

 each character is coordinated with that of every other char- 



