158 THE EVOLUTION OF MAN 



ancestors language, literature, science, property, customs, in- 

 stitutions. These are no part of our germ-plasm, nor even 

 of our blood and brain, but rather of our environment. Be- 

 cause of this social inheritance society may advance from age 

 to age, each generation starting where the preceding one 

 ended, as in a relay race, whereas in our germinal inherit- 

 ance each generation begins where the previous one began, 

 namely from an egg cell, and the whole course of development 

 must be repeated in each generation. Civilization is the re- 

 sult of the accumulations of social inheritance, and the future 

 progress of society must depend largely upon this capacity of 

 profiting by the experiences of former generations. 



Natural Selection. If mutations are the materials, natural 

 selection is the architect and director of evolution, for although 

 it does not originate fitness it continually eliminates the unfit 

 and in the long run preserves only the fit. In certain quarters 

 it has been fashionable of late to decry the importance of 

 natural selection, but more and more, biologists are coming 

 to recognize that it is the most important directing and per- 

 fecting factor in evolution. 



Natural selection in its widest meaning involves not merely 

 the overproduction of individuals and the consequent struggle 

 for existence with elimination of the unfit, as Darwin formu- 

 lated it, but it also includes the overproduction of many vital 

 activities, such as motions and reactions, with the elimination 

 of the unfit, as in the process known as "trial and error." 

 Thus useful behavior is the residue left after useless responses 

 are eliminated, and it is not necessary to hold with Darwin 

 that fitness is always the result of the elimination of unfit 

 persons. It is often the result of the elimination of unfit re- 

 actions. In short, natural selection is not only personal but 

 also intra-personal. 



Furthermore there are many kinds of fitness not only for 

 different environments but also for different organisms and 



