Xii. J THE ORIGIN OF SPECIES. 2G3 



deviate but little from a course parallel to either, or 

 to both. 



.Represent the reason of the law to ourselves by what 

 physical metaphor or analogy we will, however, the 

 great matter is to apprehend its existence and the im- 

 portance of the consequences dcducible from it. For 

 things which are like to the same are like to one another, 

 and if, in a great series of generations, every offspring is 

 like its parent, it follows that all the offspring and all 

 the parents must be like one another; and that, given 

 an original parental stock, with the opportunity of un- 

 disturbed multiplication, the law in question necessitates 

 the production, in course of time, of an indefinitely large 

 group, the whole of whose members are at once very 

 similar and are blood 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 re- 

 production, have descended from a single stock, and 

 though is is very likely they really have done so, yet 

 this conclusion rests on deduction aiid can hardly hope 

 to establish itself upon a basis of observation. And the 

 primitiveness 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 founda- 

 tion, if by " primitive " be meant " independent 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 



