3 8 REPTILES 



of reptiles, to which attention is more particularly directed in 

 the paragraphs immediately following. 



The subject of the distribution of reptilian life on the globe 

 at the present day is so extensive and complex, and involves 

 the consideration of such a number of problems, that to accord 

 it anything like adequate treatment would demand the greater 

 part of the space accorded in the present work to reptiles. 

 Obviously, therefore, only the vaguest sketch can be attempted. 



In the first place, it must be noted that, owing to the much 

 greater antiquity of reptiles as a whole than mammals or birds, 

 their dispersal, or " radiation," has taken place at a much earlier 

 date than that of either of the latter groups. This, then, would 

 be sufficient to demonstrate that the zoological " realms and 

 regions " into which the surface of the globe has been parcelled 

 out on the evidence of the present and past distribution of 

 mammals and birds would not hold good for reptiles. This, 

 however, is by no means all, for a study of the distribution of 

 the existing orders of reptiles shows that there is little in 

 common between them, and that each (doubtless owing to the 

 different dates at which the groups have come into existence) 

 has a "dispersal," or "radiation," of its own. Hence, not only 

 cannot the distribution of reptiles be brought into line with 

 that of mammals, but we cannot even map out a series of zoo- 

 logical regions for reptiles as a whole. 



Certain facts in regard to the distribution of reptiles present 

 problems which it is difficult to explain, in the present state of 

 knowledge. Taking, for example, the northern hemisphere, of 

 which the greater part is included in the Holarctic region of 

 geographical zoology, with a Palaearctic, or Old World, and a 

 Nearctic, or New World, subdivision, we find an extensive 

 community of mammalian types, due to the fact that in post- 

 glacial times there existed a circumpolar mammal fauna, the 

 members of which passed by way of which is now Bering Strait 

 from one hemisphere to the other. On the other hand, no such 

 circumpolar reptilian fauna existed in comparatively modern 

 times, for the reason that reptiles cannot withstand the rigours 

 of Arctic cold, and consequently could not pass from one hemi- 

 sphere to the other across Bering Strait, in the same manner as 

 the elk and the wapiti reached Alaska from Asia. The diffi- 

 culty in the case of the reptiles is exemplified by the alligators, 



