Isolation. 35 



and similarly with regard to longer or shorter legs, 

 wings, tails. &c., darker or lighter colour, and so on 

 through all the parts of any given organism. 



Well, although I have no wish at all to disparage 

 the biological value of these actual measurements 

 of the range of individual variation, I must point 

 out that they are without any value at all in the 

 connexion which Mr. Wallace adduces them. We 

 did not require these measurements to tell us the 

 broad and patent fact that " no being on this earthly 

 ball is like another all in all " or, in less Tenny- 

 sonian words, that as regards every specific structure 

 there is a certain amount of individual variability 

 round an average mean. Indeed, in my own paper 

 on Physiological Selection against which Mr. Wal- 

 lace is here specially arguing- I expressly said, as 

 previously remarked, " that a specific type may 

 be regarded as the average mean of all individual 

 variations" The fact of such individual variability 

 round a specific mean has always been well known 

 to anatomists ; it constitutes one of the basal pillars 

 of the whole Darwinian theory ; and is besides a 

 matter of universal recognition as regards human 

 stature, features, and so forth. The value of Mr. 

 Allen's work consists in accurately measuring the 

 amount or range of individual variation ; but the 

 question of its amount or range is without relevancy 

 in the present connexion. For the desirability of 

 isolation as an aid to natural selection even where 

 monotypic evolution is concerned, does not arise 

 with any reference to the amount or range of variation : 

 it arises with reference to the number of variations 

 which are or are not similar and simultaneous. If 

 P a 



