THE ANIMAL KINGDOM 93 



rounding conditions, may serve to open the whole 

 field of zoology. Huxley attempted to do this by the 

 use of a single animal in his famous Introduction to 

 the study of Zoology based upon the study of the 

 Crayfish in all its essential relations to other animals 

 and to nature at large. 



The study of types is a good procedure, but how 

 to avoid having this method lead merely to accumu- 

 lation of a certain set of facts regarding animals is a 

 matter of much concern, since that has been the 

 result, in so many cases, of holding exclusively to this 

 plan. As stated in an earlier chapter, what is lacking 

 in this method is concurrent attention to the back- 

 ground of the subject information about the condi- 

 tions under which the subject developed, the cir- 

 cumstances that made possible its advances, the men 

 who accomplished results and an orderly though 

 brief account of the steps in its development. 



The Number of Animals. The number of known 

 animals is ever increasing by descriptions of new 

 species. Aristotle mentioned about 500 but probably 

 knew more. Linnaeus, in 1758, described 4236 

 species. About a hundred years later, Agassiz and 

 Brown listed 129,530. In 1886, Ludwig's revision of 

 Leunis's Animal Kingdom gives 273,220, and Pratt, 



