THE 



AMERICAN NATURALIST 



Vol. LI. July, 1917 No. 607 



RATS AND EVOLUTION 

 A. C. HAGEDOORN, Med.Arts, and A. L. HAGEDOORN, Ph.D. 



In treating a large group of animals from the stand- 

 point of a systematical zoologist, it makes a very great 

 amount of difference whether one does the work in the 

 region inliabited by the animals, or somewhere else with 

 the aid of collections in a museum. A real systematist, 

 of the museum kind, does not come into touch with a 

 number of very real problems which present themselves 

 to field workers, and when he does, he has every induce- 

 ment to brush them aside with an authoritative gesture, 

 as he is not in a position to valuate their importance. He 

 takes for granted that two similar skins with similar 

 skulls which he receives from the same place, correspond 

 to a multitude of individuals, all with these same char- 

 acters ; that they are a sample of a multitude of animals 

 all exactly alike, and when he finds that animals of such a 

 description have not hitherto been named, he can invent 

 a well-sounding name for the two skins, and publish 

 a description, and henceforth this description of the type 

 specimen and this species name are welded together. If 

 it so happens that an animal is never again collected 

 which corresponds to the published description, the 

 species becomes known as very rare. 



There exist conventional rules, which, in the descrip- 

 tions of species in certain groups, ascribe more value to 

 certain characters than to others. In the systematic clas- 

 sification of rats, the points which are specially noted 

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