378 EVOLUTION OF PHYSICAL CHAEACTEKS 



perfect development of the senses of seeing and hearing, and 

 upon resistance to cHmatic conditions. We have thus to think 

 of the whole course of bodily evolution as changing in the 

 third period so as to meet a new danger rather than as con- 

 tinuing on the former lines so as better to contend with the old 

 difficulties. 



It is noteworthy that selection in this period has not been on 

 account of diseases of the second type due directly to structural 

 defects. These diseases are seldom lethal until after maturity 

 has been reached, and the tendency has rather been, as in the 

 case of eyesight, towards the increased chance of survival of those 

 suffering from these defects. Thus, though selection has turned 

 in this period towards the weeding-out of those susceptible to 

 the attacks of parasites, it has come to tolerate those who exhibit 

 defects in structure as distinguished from the peculiarities of 

 structure which must be presumed to constitute the physical 

 basis of susceptibility. 



All this is famihar enough. It is not so often realized that the 

 disappearance of polygamy in what we have called the mediaeval 

 and the modern sections of the third period works in precisely 

 the same manner. But the disappearance of polygamy does not 

 mean that reproductive selection ceases to be of importance. In 

 mediaeval and modern times celibacy, postponement of marriage, 

 and restriction of famihes have come to be practised in varying 

 degrees by different stocks. There is some trace of differential 

 fertility in the earlier periods owing to causes other than polygamy, 

 but it is only in the mediaeval and modern periods that they 

 become important. It is doubtful how far, if at all, the religious 

 celibate class of the mediaeval period differed in physical characters 

 from the average. It would appear that during this period 

 postponement of marriage led to the producing of less children 

 by the lower social classes than by the upper. Therefore, so far 

 as this epoch is concerned, it is not apparent that differential 

 fertility had any considerable effect upon bodily characters one 

 way or the other. Possibly the net result of postponement may 

 have been to favour the better stocks. 



Within the latter part of the modern period restriction of 

 families has assumed very great importance. It has been calculated 

 that, as a consequence of the fact that restriction is more practised 

 by certain sections of the population than by others, 50 per cent. 



