144 EVOLUTION AND GENETICS 



verse our present attitude and study minutely the 

 effects of each new gene on the changes that it 

 brings about in the Hfe of the individual and on its 

 23roductivity. We would then regard the superficial 

 characters as by-products of the invisible effects, un- 

 important in themselves and at best only indices of 

 internal modifications. 



Chance and Evolution 



When we consider the innumerable physiological 

 adjustments of any organism, and the many struc- 

 tural adjustments of the parts of the body to each 

 other and to the environment, an appeal to evolu- 

 tion through chance variation may seem preposter- 

 ous. Stated in this general way the theory of evo- 

 lution by chance variations seems repellent to the 

 traditional thinking of many persons. It is this sup- 

 posed difficulty, I think, that has driven some biolo- 

 gists and laymen, either to the acceptance of some 

 sort of external guiding principle responsible for 

 evolution, or to the assumption of an internal mys- 

 tical property (entelechy) of living things, or to the 

 cruder appeal to the inheritance of acquired char- 

 acters. There is, however, a well known property of 

 living organisms that puts the theory of chance, as 

 the sufficient agent in evolution, on a very different 

 footing from chance as generally imderstood. This 

 is the property of living things to multiply their 

 kind indefiniteh% i.e., to reproduce an indefinitely 



