142 DARWINISM CHAP. 



of adaptations which are usuall}^ common to many species, or, 

 more commonly, to genera and families ; but, I urge further, 

 it has not even been proved that any truly " specific " 

 characters — those which either singly or in combination dis- 

 tinguish each si)ecies from its nearest allies — are entirely un- 

 ada])tive, useless, and meaningless ; while a great body of facts 

 on the one hand, and some weighty arguments on the other, 

 alike prove that sjiecific characters have been, and could only 

 have been, developed and fixed by natural selection because of 

 their utility. We may admit, that among the great number of 

 variations and sports which continually arise many are altogether 

 useless without being hurtful ; but no cause or influence has 

 been adduced adequate to render such characters fixed and 

 constant throughout the vast number of individuals which con- 

 stitute any of the more dominant species.^ 



The Sicamjwig Effects of Intercrossing. 



This supposed insuperable difficulty was first advanced in 

 an article in the North British Review in 1867, and much 

 attention has been attracted to it by the acknowledgment of 

 Mr. Darwin that it proved to him that "single variations," 

 or what are usually termed " sports," could very rarely, if ever, 

 be perpetuated in a state of nature, as he had at first thought 

 might occasionally be the case. But he had always considered 

 that the chief part, and latterly the whole, of the materials 

 with which natural selection works, was afi'orded by individual 

 variations, or that amount of ever fluctuating variability which 

 exists in all organisms and in all their parts. Other writers 

 have urged the same objection, even as against individual 

 variability, apparently in total ignorance of its amount and 

 range ; and quite recently Professor G. J. Romanes has adduced 



^ Darwin's latest expression of opinion on this question is interesting, since 

 it shows that he was inclined to return to his earlier view of the general, or 

 universal, utility of specific characters. In a letter to Semper (30th Nov. 

 1878) he writes : " As our knowledge advances, very slight differences, con- 

 sidered by systematists as of no importance in structure, are continually 

 found to be functionally imjiortant ; and I have been especially struck with 

 this fact in the case of plants, to which my observations have, of late years, 

 been confined. Tlierefore it seems to me rather rash to consider slight 

 differences between rejjresentative species, for instance, those inhabiting the 

 different islands of the same archipelago, as of no functional importance, and 

 as not in any way due to natural selection " {Life of Darwin, vol. iii. p. 161). 



