xii.] THE ORIGIN OF SPECIES. 229 



deducible 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 undisturbed 

 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 definable as &quot; the 

 offspring of a single primitive stock.&quot; 



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 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 foundation, if by &quot;primitive &quot; 

 be meant &quot; independent of any other living being.&quot; 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 physio 

 logist who should attempt to apply it in Nature would soon find 

 himself involved in great, if not inextricable, 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, either in form or in structure. 

 There is always a certain amount of deviation, not only from 

 the precise characters of a single parent, but when, as in most 

 animals and many plants, the sexes are lodged in distinct 

 individuals, from an exact mean between the two parents. 

 And indeed, on general principles, this slight deviation seems 



