330 OUTLINES OF EVOLUTIONARY BIOLOGY 



are found only in Australasia and America, but the fossil remains 

 of such animals are widely distributed over the northern hemi- 

 sphere. The Onychophora, again, a small group of extremely 

 primitive arthropods, which until recently were all included by 

 zoologists in the single genus Peripatus (Fig. 167), are almost 

 confined to Australasia, South Africa and South America, in all 

 of which regions they are fairly abundant. It is more reasonable 

 to imagine that the ancestors of the Onychophora migrated 

 from the north, where the group has now become extinct, 

 than to invent imaginary continents 

 across which they may have wandered, 

 or even to suppose that they have 

 been so widely distributed as we now 

 find them by some external agency 

 such as floating timber. 



The geographical distribution of 

 plants and animals would be quite 

 inexplicable on the supposition that 

 they had all been independently 

 created and deposited where they 

 now live. It is, however, easy enough 

 to explain it on the theory that the 

 earth has been peopled by the des- 

 cendants of common ancestors which 

 migrated from place to place as 



FIG. 167. Peripatus capmsis, occasion permitted and at the same 



(rrom C a P p\otogr Jph X ) f ' time underwent modification in many 



different directions. We may now 



briefly summarize the principal facts of distribution which justify 

 us in holding this view. 



(1) The extent of the area of distribution of any group of 

 animals is directly proportional to its means of dispersal. Thus 

 flying animals are much more widely distributed than quadrupeds. 

 Birds occur abundantly on oceanic islands, but the only mammals 

 which occur there in a state of nature are bats and small forms 

 like rats and mice which may be carried on floating timber. 

 Nevertheless we know that when the larger mammals are trans- 

 ported by man to such localities they flourish exceedingly. 

 Many Protozoa, again, which are readily blown about in the form 

 of dust, are almost cosmopolitan even as regards their species. 



(2) The degree of peculiarity of the fauna and flora of any 



