XVII 

 THE EVOLUTION OF PHYSICAL CHARACTERS 



1. Having discussed what is given in the germinal constitution, 

 we are now in a position to inquire into the question of changes 

 in the germinal constitution. Such changes may affect physical 

 or mental characters. In this chapter we are concerned with 

 physical characters alone. Compared with the evolution of 

 mental characters the evolution of physical characters is a sub- 

 sidiary matter ; as this evolution, however, has gone hand in 

 hand with the evolution of mental characters and furthermore 

 possesses an importance of its own, all reference to it cannot 

 be omitted though the subject will be treated very briefly. As 

 a preliminary to this inquiry we may note certain facts about the 

 selection of physical characters in general. 



We know practically nothing as to the cause of mutations. 

 We have, therefore, to take mutations for granted, and to inquire 

 how certain stocks with particular predispositions are favoured 

 and others eliminated. In this process both lethal selection and 

 reproductive selection play a part. Among species in a state of 

 nature lethal selection is more important than reproductive 

 selection ; among men the importance of reproductive selection 

 tends to increase until in the latest period it assumes an impor- 

 tance approaching that of lethal selection. 



Hitherto we have only referred in passing to the existence of 

 tradition among men. Tradition will be found to form a serious 

 complication when we come to deal with the selection of mental 

 characters. With regard to physical characters it also complicates 

 the position, but only to a relatively unimportant degree. In the 

 first place it makes the environment very varied, especially in 

 industrial societies where men living next door to one another 

 may be subject to very different conditions in their daily occupa- 

 tions. Secondly, the fact that men protect themselves against 

 external conditions — for instance, against cold — may lead to 

 a group of men who are protected surviving, where a less well- 

 protected group perishes, though the former may conceivably be 



