Lharacters as Adaptive and Specific. 223 



selection. Or, to put the case in Mr. Wallace's 

 own words " This development [of useless specific 

 characters] will necessarily proceed by the agency of 

 natural selection [as a necessary condition] and the 

 general laivs which determine the production of colour 

 and of ornamental appendages? The case, therefore, 

 is just the same as if one were to say, for example, 

 that all the ailments of animals and plants proceed 

 from correlation with life (as a necessary condition), 

 "and the general laws which determine the production" 

 of ill-health, or of specific disease. In short, the 

 word " correlation " is here used in a totally different 

 sense from that in which it is used by Darwin, and in 

 which it is elsewhere used by Wallace for the purpose 

 of sustaining his doctrine of specific characters as 

 necessarily useful. To say that a useless character 

 A is correlated with a useful one B, is a very different 

 thing from saying that A is " correlated with vital 

 power," or with the general conditions to the exist- 

 ence of the species to which it belongs. So far as the 

 present discussion is concerned, no exception need be 

 taken to the latter statement. For it simply sur- 

 renders the doctrine against which I am contending. 



IV. Isolation. 



It is the opinion of many naturalists who are 

 well entitled to have an opinion upon the subject 

 that, in the words of Mr. Dixon, " Isolation can 

 preserve a non-beneficial as effectually as natural 

 selection can preserve a beneficial variation 1 ." The 

 ground on which this doctrine rests is thus clearly 



1 Nature, vol. xxxiii. p. 100. 



