DISCONTINUOUS DISTEIBUTION 333 



of the same region. Similarly in South America at the present 

 time the edentates (sloths, armadillos and ant-eaters) form the 

 most characteristic mammalian group, and the tertiary deposits 

 of that country have yielded the remains of a great number of 

 extinct forms belonging to the same order. It would be very 

 difficult to explain these facts on any theory of special creation, 

 but we can easily understand how a group of animals, having 

 once gained a footing in any area and finding itself secure and 

 more or less cut off from communication with other parts of the 

 world, would increase and vary, producing new species and 

 ultimately becoming the dominant group in that particular 

 region. 



(4) Cases of discontinuous distribution are readily explicable 

 on the theory of evolution and migration. Either individuals 

 of the species in question have occasionally transgressed the 

 barriers to their dispersal and established new and distant 

 colonies, or possibly a large area of distribution has become 

 broken up into a number of smaller ones by geographical or 

 climatic changes rendering portions of it uninhabitable. 



" Thus, for instance," says Romanes, " it is easy to understand 

 that during the last cold epoch the mountain hare would have 

 had a continuous range ; but that as the arctic climate gradually 

 receded to polar regions, the species would be able to survive in 

 southern latitudes only on mountain ranges, and thus would 

 become broken up into many discontinuous patches, correspond- 

 ing with these ranges. In the same way we can explain the 

 occurrence of arctic vegetation on the Alps and Pyrenees 

 namely, as left behind by the retreat of the arctic climate at the 

 close of the glacial period." 1 



1 " Darwin and after Darwin," Vol. I., p. 209. 



