36 ESSAYS OF A BIOLOGIST 



There will thus be a constant biological pressure 

 (to use a term which, though still symbolic, a mere 

 analogy, is less misleading and question-begging than 

 elan vital) tending to push some of life on to new 

 levels of attainment, new steps in progress, because 

 any variations in that direction will have selection 

 value, a selection value above the ordinary. And 

 the process will be a gradual one, because variations 

 are not very large ; so that life no more realizes all 

 potentialities of progress at once than did the United 

 States or any other new country receive a uniform 

 population over all its extent as soon as it was dis- 

 covered, but had its people move in from the coasts 

 in a regular and orderly advance. 



There are plenty of parallels from human affairs. 

 Indeed, the evolutionist can often gain valuable light 

 on his subject, on what one may call the economics 

 of the process, by turning to study the development 

 of human inventions and machines. There, although 

 the ways in which variations arise, and the way they 

 are transmitted, are different from those of organic 

 evolution, yet the type of * pressure,' the perpetual 

 struggle, and the advantage of certain kinds of varia- 

 tion therein — these are in essence really similar. 



What could be more striking than the parallel 

 between the rise of the mammals to dominance over 

 the reptiles, and the rise of the motor vehicle to 

 dominance over that drawn by horses ? 



In both cases, a comparatively long period in which 



