II THE ORIGIN OF SPECIES 33 



relations, having descended from the same parent, 

 or pair of parents. The proof that all the members 

 of any given group of animals, or plants, had thus 

 descended, would be ordinarily considered sufficient 

 to entitle them to the rank of physiological species, 

 for most physiologists consider species to be de- 

 finable as "the offspring of a single primitive 

 stock." 



But though it is quite true that all those 

 groups we call species may, according to the 

 known laws of reproduction, have descended from 

 a single stock, and though it is very likely they 

 really have done so, yet this conclusion rests on 

 deduction and can hardly hope to establish itself 

 upon a basis of observation. And the primitive- 

 ness of the supposed single stock, which, after all, 

 is the essential part of the matter, is not only a 

 hypothesis, but one which has not a shadow of 

 foundation, if by " primitive " be meant " indepen- 

 dent of any other living being." A scientific 

 definition, of which an unwarrantable hypothesis 

 forms an essential part, carries its condemnation 

 within itself; but, even supposing such a 

 definition were, in form, tenable, the physiologist 

 who should attempt to apply it in Nature would 

 soon find himself involved in great, if not in- 

 extricable, difficulties. As we have said, it is 

 indubitable that offspring tend to resemble the 

 parental organism, but it is equally true that the 

 similarity attained never amounts to identity 



