40 evolution: the ages and tomorrow 



and their complement of genes from two different individ- 

 uals who are very unlikely to possess identical hereditary 

 sets. Thus, there is in nature not only the means by which 

 new hereditary determiners may appear, but also a mech- 

 anism by means of which they may be shuffled and distrib- 

 uted. 



The problem of applying all these factors to the process 

 of evolution is enormously complex and difficult, but at no 

 time is there ever any justification for the use of any mech- 

 anism in explanation which does not operate through the 

 genes and chromosomes. The forces involved act on in- 

 dividual organisms, but the effects work out within asso- 

 ciated groups of individuals or populations, and over great 

 stretches of time. In recent years a new science of popula- 

 tion genetics has been developed by J. B. S. Haldane, R. A. 

 Fisher, and Sewall Wright, employing its own language and 

 mathematics and dealing in a much more promising way 

 with the evolutionary complex. 



The older idea of "struggle" and "survival" was too in- 

 dividualized. In the early post-Darwinian days the phrases 

 "the struggle for existence" and "the survival of the fittest" 

 were overemphasized— the lone animal dying in the losing 

 battle with its enemy or rival. It was never the intention of 

 Darwin or his scientific followers, however, that these 

 phrases should find their way into ethics and politics. Nor 

 did Darwin think only of "fitness" as physical prowess and 

 pugnaciousness. He knew that factors must be carefully 

 analyzed in any given case and that the helpfulness of one 

 animal for another of its kind (mutual aid) was a part of 

 fitness. Struggle, survival, and fitness on an individual basis 

 do not apply in a too literal sense of the words. Natural 

 selection favors very broadly those individuals having the 

 most offspring, which usually means those best adapted to 

 a particular habitat in which they find themselves. It is 

 obvious that adaptation is approximate, never perfect. Ad- 

 aptation is the orienting principle which guides the random 



i 



