388 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



Isolation is also produced by inequalities of land- 

 surface, resulting from the elevation of mountain 

 ranges, plateaus, etc. This is well seen in Mexico 

 and Central America where the number of species of 

 Vertebrata is large, owing to the fact that many of 

 them are restricted to very narrow areas bounded by 

 impassable barriers. The restriction of species of land 

 Mollusca to each of the numerous valleys of the Ha- 

 waiian Islands has been made the subject of an espe- 

 cial study by Dr. Gulick, who treats of them with 

 especial reference to the evolution of their forms. 



The assimilation of inorganic matter necessarily 

 preceded that of organic matter, so that this function 

 characterized the first organic beings, whether animal 

 or plant-like in other respects. Among animals we 

 may regard the vegetable feeders as having by a little 

 preceded the carnivorous forms. Omnivorous forms 

 must have come into existence soon after, and from 

 these, both classes of feeders have been from time to 

 time recruited ever since. The primitive Vertebrata 

 were probably carnivorous, and most of the fishes and 

 Batrachia have always been such. Herbivorous forms 

 have arisen from time to time among Reptilia, and of 

 granivorous birds there are many. The early Mamma- 

 lia were divided between omnivorous (Multitubercu- 

 lata) and insectivorous types (Protodonta, Pantothe- 

 ria) ; while the higher Mammalia of all kinds were de- 

 rived from more or less omnivorous forms (primitive 

 Condylarthra and some Creodonta). We may account 

 primitive insects to have been largely herbivorous, even 

 more than they are at the present time, while Carnivora 

 predominate in marine invertebrate life. It is not diffi- 

 cult to understand that circumstances of the environ- 

 ment would determine the food of animals, and would 



