126 PERIOD V. 



been unconsciously seeking, and not some unknown 

 plan of creation, or the enunciation of general proposi- 

 tions, and the mere putting together and separating 

 objects more or less alike." 1 



Natural groups, large or small, result from the long- 

 continued operation of divergence, the survival of 

 some, and the extinction of others ; they are to be 

 respected as facts ; they are not created by definitions, 

 which only serve to indicate and remind ; any character, 

 however trifling, will suffice, if only it is constant and 

 distinctive. 



The conflict between natural classification and logic 

 is apparent only. Logicians say that in classifying 

 books, for instance, you may take any property you 

 please, subject, size, etc., as the basis of your arrange- 

 ment, but having made your choice, you must adhere 

 to it for all divisions of the same rank. Naturalists 

 seem to say something different, for they are agreed 

 that what they call "single-character classifications," 

 in which one property is adhered to throughout, are 

 unnatural. The fact is that a natural classification 

 always rests upon one and the same property viz. 

 affinity p , i.e. relative nearness of descent from some 

 common ancestor. Every natural classification, like 

 every logical classification, proceeds upon a single basis, 

 and the failure of the single-character classifications 

 is due to their displacing affinity by some definition. 



The effect of the Origin of Species upon zoological 

 and botanical systems has been revolutionary. Fur- 

 nished with a new and intelligible meaning of the word 

 natural, and with new criteria of naturalness, syste- 

 matists have during the last fifty years worked hard to 



1 Origin of Species, chap. xiii. 



