300 THE EEGULATION OF NUMBEKS 



numerous migrations of which we have knowledge, there are but 

 few cases concerning which we have detailed information of the 

 social conditions. But where this information does exist, in no 

 instance do we find evidence that migration began after a con- 

 dition of over-population had come about. 



It may be that what is implied by those who put forward the 

 view that migration is somehow connected with over-population 

 is somewhat different. It may be implied that migration occurs 

 when what we have called the optimum number has been attained. 

 If our general conclusions on this subject are borne in mind, it 

 is clear that something more than this is required as an explana- 

 tion. Migration occurs irregularly at long intervals ; adjustment 

 of population to some point approximating to the desirable level 

 is always going on. Clearly it cannot be in the ordinary process 

 of adjustment that some impetus to migration occurs ; there must 

 be some particular predisposing factor or factors at work. What are 

 these factors ? It would seem to rest with those who put forward 

 the theory to show what they are, and this has not been done. 



On the other hand, there is another explanation which appears 

 to account at least in part for nearly every migration of which 

 we have some knowledge of the circumstances, and it may be 

 that an explanation on similar lines accounts at least in part 

 for most of the later migrations in history. There is nothing 

 novel in this explanation ; it is admitted as an explanation of 

 many migrations by Mr. Haddon. 1 He admits that many migra- 

 tions have been undertaken for political or religious reasons and 

 instances the voyage of the Mayflower and the Islamic and 

 Buddhistic movements. That which is common to these explana- 

 tions is that migration is undertaken in response to some idea. 

 There is no question of over-population at all, and in all those 

 cases in which we have any detailed knowledge of the conditions 

 connected with migration we find that we can point to some 

 idea as the motive force. Why, therefore, when details are not 

 known, should another explanation be sought ? It is suggested 

 that essentially the same explanation is the most reasonable 

 explanation of all migrations, at least in historical times. With 

 regard to the earlier movements we find that migrations appear 

 to occur where a high level of skill has been reached in some 

 area. Where races are in contact with others of a markedly 

 1 Haddon, Wanderings of Peoples, p. 3. 



