CH. I] DISCONTINUOUS DISTRIBUTION. 15 



a level tract of country with no trees; nor can an 

 Amphibian whose skin requires to be kept moist cross an 

 arid desert. This consideration leads to an important 

 matter, the capacities for migration possessed by different 

 animals which will be discussed later. 



Discontinuous distribution. 



On the hypothesis that each animal has had its centre 

 of dispersal, that it came into existence once and at a 

 definite place, it is clear that originally at least the area 

 inhabited by a given species must have been perfectly 

 continuous. As a matter of fact it is generally the case ; 

 the remarkable thing appears to be not that there are 

 occasionally breaks in the continuity of the area inhabited 

 by a certain species but that it is so difficult to find 

 instances to illustrate the breaks. Mr Wallace explains 

 the rarity of discontinuous distribution among the species 

 of birds by the suggestion that they are possibly "more 

 rapidly influenced by changed conditions, so that when a 

 species is divided the two portions almost always become 

 modified into varieties or distinct species." It must be 

 borne in mind also that birds are a modern group, and the 

 very difficulty of classifying them satisfactorily indicates 

 that there are but few breaks in the series; they are 

 possibly still in a condition of perpetual modification; 

 they have not so to speak become fixed and crystallised, 

 like some of the older and in a sense more effete groups of 

 animals. However this may be, Mr Wallace quotes from 

 Mr Seebohm a highly remarkable instance of discontinuous 



