ORGANIC EVOLUTION — MENTAL 173 



of physical structures profoundly modified in conse- 

 quence of the acquirement by generation after genera- 

 tion, through innumerable generations, of certain 

 mental traits ; we should have an example of physical 

 structures profoundly modified, because the traits ac- 

 ([uired caused the survival of individuals different from 

 those which would otherwise have survived. In that case 

 the acquired habit of slave-making, transmitted from 

 preceding generations to succeeding generations, as men 

 transmit language or property, has been the cause of 

 the survival of those ant communities in which the 

 individuals were most fit mentally and structurally for 

 slave-makinsf, for fifjhtino- rather than of those com- 

 munities in which the individuals were most fit for 

 maintaining existence by their own industry, and there- 

 fore the cause of the evolution of the enormous jaws 

 and their co-ordinated structures, and the concomitant 

 retrogression of many other structures. 



In man occur many examples of structural evolution 

 and retrogression traceable to the persistent acquire- 

 ment by generation after generation through many 

 generations of various traits. His hearing and notably 

 his sense of smell have retrogressed, because, owing to 

 his growing powers of acquiring reason, these senses 

 less and less influenced the survival rate. His teeth, 

 and all the structures co-ordinated with them (jaw-bones, 

 muscles, nerves, blood-vessels, &c.), have retrogressed, as 

 has also his whole digestive apparatus, owing to his 

 acquired habit of cooking his food ; the survival rate 

 being here beneficially influenced by an acquired trait, 

 which rendered him above all other animals omnivorous, 

 which enabled him to use for food a greater number of 

 things than any other animal ; and therefore, though 

 man is no longer able to digest the raw food on which 

 his remote ancestors subsisted, yet his acquired habit 

 brings him into completer harmony with the environ- 



