CHAP, xxm.] SUMMARY AND CONCLUSION. 547 



are examples of the first-named peculiarity ; the Azores and the 

 Bermudas of the last ; and the difference can be clearly 

 traced to the frequency and violence of storms in the one case 

 and to the calms or steady breezes in the other. 



It appears then, that although birds do not afford us the same 

 convincing proof of the former union of now disjoined lands as 

 we obtain from mammals, yet they give us much curious and 

 suggestive information as to the various and complex modes in 

 which the existing peculiarities of the distribution of animals 

 have been brought about. They also throw much light on the 

 relation between distribution and the external characters of 

 animals ; and, as they are often found where mammalia are quite 

 absent, we must rank them as of equal value for the purposes of 

 our present study. 



Eeptiles. 



These hold a somewhat intermediate place, as regards their 

 distribution, between mammals and birds, having on the whole 

 rather a wider range than the former, and a more restricted 

 one than the latter. 



Snakes appear to have hardly more facilities for crossing the 

 ocean than mammals ; hence they are generally absent from 

 oceanic islands. They are more especially a tropical group, and 

 have thus never been able to pass from one continent to another 

 by those high northern and southern routes, which we have seen 

 reason to believe were very effectual in the case of mammalia 

 and some other animals. Hence we find no resemblance between 

 the Australian and Neotropical regions, or between the Palae- 

 arctic and Nearctic; while the Western Hemisphere is com- 

 paratively poor as regards variety of types, although rich in 

 genera and species. Deserts and high mountains are also very 

 effectual barriers for this group, and their lines of migration have 

 probably been along river valleys, and occasionally across narrow 

 seas by means of floating vegetation. 



Lizards, being somewhat less tropical than snakes, may have 

 passed by the northern route during warm epochs. They are also 

 more suited to traverse deserts, and they possess some unknown 



