No. 495] 



WHAT IS A SPECIES? 



191 



mately upon structure. Two cells absolutely alike must 

 doubtless function quite alike. But I doubt if there ever 

 are any two cells quite alike. We are already learning, if 

 I am correct in my understanding of the claims of Mc- 

 Clung and others, that even minor, so-called specific dif- 

 ferences are discoverable in the cell, in some groups at 

 least. There is, of course, no such thing as a purely phys- 

 iological species, for changed structure must underlie all 

 variation, though we may not be able to discover the 

 differences. I can conceive that additional or modified 

 chromosomes in the germ cells of the greyhound might 

 indicate a partial physiological isolation which has pre- 

 served this race of dogs almost undefiled through more 

 than three thousand years ; certainly man has not been the 

 cause of its preservation. 



Now, if the foregoing theses or hypotheses be true, or 

 even if the greater part of them be fundamental principles 

 of variation, it follows that the definition of species must 

 be made for each and every one that exists or has existed ; 

 that a specific character in one group may be merely 

 varietal in an allied group, on one hand, or generic in 

 another, on the other hand. Or, aphoristically, every spe- 

 cies, as we know to be the case with every genus, is a law 

 unto itself. And this is, practically, the working rule of 

 every competent taxonomist, though of course many sad 

 errors are made in its application. And it follows that he 

 who is best qualified to propose and name species, or to 

 criticize those which have been proposed by others, is one 

 whose acquaintance is widest with living forms and with 

 the laws which underlie their evolution. He must not con- 

 found genetic with adaptive characters, for phylogeny is 

 the sole end of taxonomy. 



Since we can not give an answer to the question, What 

 is a species? let us analyze briefly some definitions of 

 the past : 



1. A species is a form of life which breeds true to itself. 

 The Jewish race has bred true to itself, as indicated by its 

 distinctive physiognomy, since the time of Rameses ; and 



