426 BOSS ox EVOLUTION. 



than that for a particular variety of food , since some exist on almost 

 every variety, while in the Flying Squirrel, and Galeopithecus the 

 adaptability for flight seems of a much less fundamental character. 



There seems to have been a steady increase in the size of 

 the larger animals of each succeeding grade, corresponding to the 

 increasing induration of tissue ; in the water from the Selachian or 

 Shark of the Upper Silurian to the hugh Cetaceans of the Recent 

 Period ; and on land from the LahyrintJiodontia of the Devonian 

 to the Recent Mastodons. The larger animals of each grade seem 

 to have been exterminated by the larger animals of the succeeding 

 higher grade, these having the advantage in the struggle for life in 

 respect of intelligence, activity, strength and ultimately even of 

 bulk. Thus the largest types of each grade, except the highest, 

 have been constantly and successively in every sense undergoing 

 extermination, so that, as we go downwards in grade, we find the 

 existing representatives smaller until we reach the Protozoa where 

 they are mostly Microscopic, although when each grade was in 

 maximum it had representatives comparable in size, though not 

 quite equal, to the largest of the succeeding grades. Now as we 

 have seen that directions taken by the diiferent Orders of each 

 Grade, have been approximately parallel or similar, each to each, 

 and as the lower and earlier grade had begun to differentiate 

 soonest it is plain that only its more differentiated types would be 

 well out of reach of this competition of the higher, and that thus the 

 less differentiated types of the lower would be constantly and 

 successively undergoing extermination, and thus only the most 

 differentiated types continue to exist, except when the more synthetic 

 types are preserved, by isolation from the access of types of a 

 higher grade, or by a difficulty of access arising from any other 

 cause. 



The Great Continent, particularly the northern Grand Division 

 of it, Asia-Europe, has been during the later Tertiary, the Post 

 Tertiary and Recent Epochs the theatre par excellence of progress 

 in every organic type, which is represented there. It was not 

 always so, however for North America in the Eocene seems, both 

 in regard to its Plants and Animals, to have reached a stage only 

 reached in Europe and Asia in the Miocene, no doubt by a migra- 



