50 THE MAMMALIA. 



often been disturbed by mighty upheavings and 

 sinkings. 



The connection of the oceans must, in fact, 

 never be altered to account for the migration and 

 distribution of the animals. For instance, it is 

 now an established fact from deep-sea investiga- 

 tions that, since the chalk period at least the 

 bottom of the sea has experienced only unimportant 

 changes, changes that are almost imperceptible in 

 their slowness and their effect upon the animal 

 world ; its petrography has, in fact, undergone such 

 small changes that it may be said that we are still 

 in the chalk period, and that the formation of 

 chalk is still proceeding. And further, we may 

 assume the process to have been the same with all 

 the other and earlier geological periods. This 

 theory may, moreover, with certain limitations, 

 be applied to the main land. Larger accumula- 

 tions of land of some consistency are probably first 

 perceived in the Coal formation, and there can be 

 no question of continents, in our present sense of 

 the word, till the Jura and Chalk periods. At all 

 events, however, temporary connections of large 

 Jura islands probably also the accumulations of 

 land belonging to the Trias must have also existed. 

 For, not merely have we to date the individual 



