ME. G. J, EOMANES ON PHTSlOLOaiCAL SELECTIOIT. 385 



ment of bristles as much too specialized a structure to be wholly 

 meauiugless. Tet we happen to know that this particular 

 structure is wholly meaningless. What, then, are we to say to 

 the argument from ignorance in other and less cogent cases ? 

 I think we must say that the argument is wholly untrustworthy 

 in fact, while even in theory it can only stand upon the assump- 

 tion (latterly discarded even by Darwin himself) that all specific 

 differences must be due to natural selection. 



Aegtjment teom Diveegence of Chaeactee. 



Any theory of the origin of species in the way of descent 

 must be prepared with an answer to the question, Why have 

 species multiplied ? How is it that, in the course of evolution, 

 species have not simply become transmuted in linear series 

 instead of ramifying into branches ? This question Mr. Darwin 

 seeks to answer " from the simple circumstance that the more 

 diversified the descendants from any one species become in struc- 

 ture, constitution, and habits, by so much will they be better 

 enabled to seize on many and widely diversified places in the 

 economy of nature, and so be enabled to increase in numbers."* 

 And he proceeds to illustrate this principle by means of a diagram, 

 showing the hypothetical divergence of character undergone by 

 the descendants of seven species. Thus, he attributes divergence 

 of character exclusively to the influence of natural selection. 



Now, this argument appears to me unassailable in all save one 

 particular ; but this is a most important particular : the argument 

 wholly ignores the fact of intercrossing with parent forms. 

 Grranting to the argument that intercrossing with parent forms 

 is prohibited, and nothing can be more satisfactory. The argu- 

 ment, however, sets out with showing that it is in limited areas, 

 or in areas already overstocked with the specific form in question, 

 that the advantages to be derived from diversification will be 

 most pronounced. Or, in Mr. Darwin's words, it is where they 

 " jostle each other most closely " that natural selection will set a 

 premium upon any members of the species which may depart 

 from the common type. Now, inasmuch as this jostling or over- 

 crowding of individuals is a needful condition to the agency of 

 natural selection in the way of diversifying character, must we 

 not feel that the general difficulty from intercrossing previously 



* ' Origin of Species,' ed. 6, p. 87. 

 LINif. JOUEN. — ZOOLOGY, VOL, XIX. 30 



