HISTORY OF ZOOLOGY. 25 



Cuvier proved beyond a doubt that these fossils were the 

 remains of animals of a previous time. Just as the forma- 

 tion of the earth's crust by successive overlying layers 

 made possible the discrimination of different natural periods 

 of the earth's history, so Paleontology taught how to 

 recognize also the different periods in the vegetable and 

 animal world of life on our globe. Each geological age 

 was characterized by a special world of animals quite 

 peculiar to it ; each period showed a successively higher 

 development of the animal world, i.e., in the older periods 

 of the earth's history only the lower animals existed, 

 but with each new period higher animals appeared, cul- 

 minating in the present period in man, the crowning 

 glory of creation. All these generalizations led Cuvier to 

 his cataclysm theory, that a great revolution brought to 

 an end each period of the earth's history, destroying all 

 life, and that upon the newly-formed virgin earth a new 

 organic world of constant species sprang up. 



Objections to the " Cataclysm Theory."- -By the sup- 

 position of numerous acts of creation the Linnean concep- 

 tion of species seemed to be rescued, though, to be sure, by 

 summoning to its aid hypotheses which had neither founda- 

 tion in natural history nor justification in theology. The 

 logical following out of Cuvier' s cataclysm theory led to 

 conceptions of a Creator who built up an animal world 

 only for the purpose of destroying it after a time as a 

 troublesome toy ; it has therefore at no time found warm 

 supporters, at least among geologists, for whom it was in- 

 tended. Of the prominent zoologists there is only to be 

 mentioned Louis Agassiz, who till the end of his life re- 

 mained faithful to this theory. 



Under these conditions it is readily understood how 

 thinking naturalists, who felt the necessity of explaining 

 the character of organic nature simply and by a natural 

 law capable of general application, began to doubt the 

 fixity of species, and were led to the theory of change of 

 form, the Theory of Descent. 



