ort 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 
VOL, II D 
