CHAP. XXIII.] 
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. 
Reptiles . 
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 Paisa- 
arctic and Neurotic; 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 
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