THE GROWTH OF GROUPS 7 



individual of his species. There is no essential difference 

 between these unobtrusive characters and the more 

 obvious ones chosen. Some characters are visible to 

 the naked eye ; others come into view when we use a 

 hand lens ; others again can only be seen properly with 

 the help of a microscope. Thus in appearance they are 

 gradually less and less perceptible. Those that are 

 scarcely perceptible cannot as a rule become more evident 

 by accumulation. We also know that specific differences 

 exist beyond the point where they cease to become 

 visible with the help of the microscope. The precipitin 

 test has shown us that there are specific differences in the 

 quality of the living substance. Therefore, in describing 

 a species we must close our description at a point. 

 Microscopic characters are often considered negligible. 

 The only assertion implied in the definition of a species 

 is that all the characters enumerated are present in every 

 individual of it. The systematist then, must overlook 

 certain characters because they are difficult to appreciate, 

 although they may be present in every individual of 

 the species he is defining. But, besides this, he delibe- 

 rately overlooks certain perceptible and perhaps obvious 

 characters, because they are not present in every indivi- 

 dual of the species as he has chosen to define it, but only 

 in some individuals. Such negligible characters are 

 said to be individual and not specific, but none the less 

 they may be perceptible and heritable attributes, although 

 they are not present in every member of the species as 

 defined. 



What is the usual fate of a species X which has been 

 defined in terms of certain chosen characters A, B, C ? 

 Let us suppose that it has been defined by a naturalist 



