GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 487 



world seems to stand still, and organic forms seem to be 

 permanent. Geology alone shows us the world in con- 

 tinuous motion, in continuous change by evolution. 



In the new and now universally adopted view there 

 are four principles to be borne in mind : 1. The origin 

 of organic forms "by descent with modifications" — i.e., 

 by evolution — and the steady forward march of evolu- 

 tion everywhere and through all time. If this were all, 

 there would be no geographical diversity at all. 2. In 

 different places, with different environments and isolated 

 from one another, evolution took different directions, and 

 faunas became more and more different as long as the 

 isolation continued. If this had been all, geographical 

 diversity would by this time have been far more extreme 

 than we find it anywhere. 3. But from time to time, at 

 long intervals in geological history, there occurred wide- 

 spread changes in physical geography and climate by 

 which barriers were removed and new barriers set up. 

 The result was wide migration and mingling and con- 

 flict of faunas, and consequently more rapid evolution, 

 but at the same time a decrease of geographical diver- 

 sity. 4. A re-isolation in new localities, and a commenc- 

 ing process of divergence which is still going on. 



Now, the last of these periods of great changes, cli- 

 matic and geographic, and of extensive migrations and 

 minglings and conflict of faunas, and therefore of rapid 

 evolution, was the glacial epoch, or ice age. It is evi- 

 dent, then, that the geographic and climatic changes of 

 the glacial epoch furnish the key to the present distri- 

 bution of species, and, conversely, the present geo- 

 graphic distribution of species furnishes a key to the 

 direction, of migrations of that time. 



A full discussion of this interesting subject should 

 be preceded by a course in geology, but an outline may 

 be brought out by means of illustrative examples. 



