222 THE EEGULATION OF NUMBERS 



who use various means to procure abortion and use them success- 

 fully. ... A considerable period elapses between the birth of one 

 child and the birth of another. The general term is about three 

 years. One child is always well out of hand before another 

 appears.' ^ We may also note that, according to Rivers, infanti- 

 cide is frequent among the Todas and is practised not when food 

 runs short but as a regular custom.^ 



It will be remembered that, when setting out in the last two 

 chapters the evidence for the extent of abortion and of infanticide, 

 these practices were in many cases stated to be committed when 

 there was a certain number of children thus corroborating the 

 evidence given to this effect above. 



9. We are now in a position to discuss the manner in which 

 numbers are regulated among these races, leaving until later the 

 discussion as to how the position among these races was derived 

 from that among species in a state of nature. Everywhere groups 

 of men are, as we have seen, confined to definite areas. Among 

 the few things which the men composing these groups do know 

 with accuracy are the limits within which their food must be 

 obtained. Further they co-operate in the search for food. It 

 follows, therefore, that within any such area there is — taking all 

 the relevant facts into account — an optimum number. The 

 advantages to any group of approaching this number are immense ; 

 a wide departure from this number can only be socially disastrous. 

 We found that within any group there is a number of factors — 

 some of which reduce fertility and others of which increase elimina- 

 tion — and that the average amount of restriction of increase 

 resulting from the action of these factors was fairly conbtant. We 

 also found that there were certain other factors — prolonged 

 abstention from intercourse, abortion, and infanticide — which are 

 everywhere present and considerably restrict increase. If, as we 

 shall see later, there is reason to think that some approach to the 

 optimum is everywhere attained, it is clear that the former factors 

 cannot of themselves alone sufficiently restrict increase. It is 

 therefore to the latter factors, the primary function of which it is 

 to restrict increase, that we must look when we seek for the 

 mechanism by which numbers are brought near the desirable level. 



It is clear how these factors originate. Among more or less 

 nomadic peoples abortion and infanticide are practised because 



1 Danks, loc. cit., p. 291. ^ Rivers, Todos, p. 401. 



