THE ORIGIN OF SPECIES 149 



but extremely complex bundles of forces held in a mass 

 of matter, as the complex forces of a magnet are held in 

 the steel by its coercive force ; and, since the differences 

 of sex are comparatively slight, or, in other words, the sum 

 of the forces in each has a very similar tendency, their 

 resultant, the offspring, may reasonably be expected 

 to 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 importance of 

 the consequences 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, w r ould be ordinarily 

 considered sufficient to entitle them to the rank of physio- 

 logical species, for most physiologists consider species to 

 be definable 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 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 " primitive " he 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 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, 



