THE SLEEPING SICKNESS 185 



the natural world the inhabitants of areas bounded 

 by sea, mountain, and river become adjusted to one 

 another ; and a balance is established. The only dis- 

 turbing factors are exceptional seasons, unusual cold, 

 wet, or drought. Such recurrent factors may from time 

 to time increase the number of the weakly who are 

 unable to cope with the invasions of minute destructive 

 parasites, and so reduce, even to extermination the kinds 

 of animals or plants especially susceptible to such influ- 

 ences. But anything like the epidemic diseases of para- 

 sitic origin with which civilised man is unhappily familiar 

 seems to be due either to his own restless and ignorant 

 activity or, in his absence, to great and probably 

 somewhat sudden geological changes changes of the 

 connexions, and therefore communications, of great land 

 areas. 



It is abundantly evident that animals or plants which 

 have, by long aeons of selection and adaptation, become 

 adjusted to the parasites and the climatic conditions and 

 the general company (so to speak) of one continent may 

 be totally unfit to cope with those of another; just as 

 the Martian giants of Mr. H. G. Wells, though marvels of 

 offensive and defensive development, were helpless in the 

 presence of mundane putrefactive bacteria and were 

 rapidly and surely destroyed by them. Accordingly, it 

 is not improbable that such geological changes as the 

 junction of the North and South American continents, of 

 North and South Africa, and of various large islands and 

 neighbouring continents, have, in ages before the advent 

 of man, led to the development of disastrous epidemics. 

 It is not a far-fetched hypothesis that the disappearance 

 of the whole equine race from the American continent 

 just before or coincidently with the advent of man a 

 region where horses of all kinds had existed in greater 



