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THE AMERICAN NATURALIST [Vol. XLII 



stable as de Vries assumed. In some characters, like coat 

 colors, fixity is more or less realized; but in other fea- 

 tures, particularly dimensional ones, such an amount of 

 variation is shown that the resulting forms necessitate the 

 same artificial methods of separation as those to which 

 the older systematists were obliged to resort. From a 

 radical standpoint there seems to be no escape from the 

 view that a species is at best an imperfectly definable 

 aggregate of more or less similar individuals and that 

 this conception rests upon an artificial basis. 



Although the idea of species must be admitted to be 

 artificial rather than natural, its immense practical im- 

 portance is not to be lost sight of ; for, as a tool in the 

 hands of the working zoologist, it is absolutely indispen- 

 sable. Moreover, the arrangement of species into related 

 groups and the development thereby of a system of classi- 

 fication has led to views on animal phylogeny of the ut- 

 most significance. That living animals range from the 

 relatively simple unicellular protozoans to the immensely 

 complex vertebrates suggests at once that the modern 

 faunas include not only the latest products of animal 

 evolution but many remnants of the remote past. This 

 state of affairs is a continual temptation to picture the 

 past history of the animal kingdom in terms of modern 

 forms and to assume, for instance, that the protozoans 

 of to-day are like the primitive protozoans from which 

 the higher animals have been derived. This attitude leads 

 to the discussion of the so-called affinities of the modern 

 groups, a practise that in my opinion is open to the 

 objection of attempting to make parents and grandparents 

 out of brothers and sisters. How futile this practise is 

 can be seen in such a group as the reptiles. You are 

 doubtless aware that the modern reptiles are included in 

 four orders : the chelonians, or turtles ; the crocodilia ; the 

 rhynchocephalia, represented by a lizard-like reptile from 

 New Zealand; and the squamata, or lizards and snakes. 

 You are probably not so well aware that the class of 

 reptiles also includes five other orders, all fossil, among 



