302 THE EEGULATION OF NUMBEES 



has been cast upon the ' pulsatory ' nature of such changes as 

 have taken place. Whether this view as to climatic change is 

 correct or not is of no particular importance for our present 

 purpose ; for on the one hand it has been shown that certain 

 migrations, particularly those in Central Asia, where the evidence 

 as to the pulsatory nature of the change is said to be most clear, 

 are explicable as due to political changes,^ while on the other 

 hand it does not appear to be possible in general to correlate the 

 migrations of history, which are essentially rapid movements, 

 with slow changes of climate. It seems likely that population 

 would adjust itself to slow climatic changes without difficulty or 

 with as little difficulty as it adjusts itself to other changes which 

 alter the optimum density. Extreme climatic changes very slowly 

 brought about, as doubtless took place in pre-history, might 

 result in driftings,^ but the less extreme changes in climate during 

 the historical period have in all probability been without any 

 pronounced effect upon movement. 



Enough has been said to show that, whereas there is much 

 that can be adduced both on theoretical grounds and after a 

 review of the evidence against the theory attributing migration 

 to over-population, there is much to recommend the extension of 

 the explanation already admitted in many cases. But even if 

 migration is not a result of over-population, it may seriously 

 affect the adjustment of population, once it has been set on 

 foot. And it is to the observation of the secondary disturbing 

 effect of migration on population that we may attribute the error 

 of tracing migration to over-population. 



When, however, all allowances are made, the easy manner in 

 which the common theory of migration is assumed to be true and is 

 used by writers of great authority is very surprising. Mr. Hogarth, 

 for instance, in a well-known and very delightful book alluding 

 to the Chaldean or fourth great wave of migration from Arabia, 

 is led to explain all these Arabian migrations as follows : ' The 

 great Southern Peninsula ', he says, ' is for the most part a high- 

 land steppe endowed with a singularly pure air and an uncon- 

 taminated soil. It breeds, consequently, a healthy population 

 whose mortality, compared with its death-rate, is unusually 

 high ; but since the peculiar conditions of its surface preclude 



* Peisker, Cambridge Mediaeval History, vol. i, p. 328. ^ Frequently, no 



doubt, such driftings were directed towards regions previously uninhabited and 

 uninhabitable. 



