NATURAL HISTORY-NINETEENTH CENTURY. 641 



by naturalists in general without a single eminent exception. How 

 profoundly this theory has modified biological conceptions may be 

 illustrated by contrasting the older definitions of species and genera 



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with those of the present day. Linnaeus defined Ins notion of species 

 by saying, " There are as many different species as there were different 

 forms created in the beginning by the Infinite Being." Cuvier, who 

 thought a science of living things impossible unless based on the im- 

 mutability of species, gave a more practical definition of species by 



41 



