658 Studies in the Theory of Descent. 



with the fact that the part concerned is longer 

 than in other individuals. 6 



But if qualitative variations come into con- 

 sideration, it may be asked whether Darwin's 

 "supplementary admission" does not go too far. 

 Such calculations as those quoted by Darwin 

 from the article in the North British Review of 

 March 1867 are extremely deceptive, since we 

 have no means of measuring the amount of pro- 

 tection afforded by a useful variation, and we can 

 therefore hardly compute with any certainty, in 

 how great a percentage of individuals a change 

 must contemporaneously occur in order to have a 

 chance of becoming transferred to the following 

 generation. If our blue rock-pigeon could exist 

 in a polar climate, and if we had the power of 

 introducing it gradually, but not suddenly, into 

 these regions in a wild state, who can doubt 

 that it would assume the white colour of all 

 polar animals ? Nevertheless, among wild rock- 

 pigeons white varieties do not occur more fre- 

 quently than among swallows, crows, or magpies. 

 Or must the white colour of polar animals, the 

 yellow colour of desert species, and the green 



6 [Mr. A. R. Wallace, in his article last referred to, quotes 

 some most valuable measurements of mammals and birds, 

 showing the amount of variation of the different parts. These 

 observations were published by J. A. Allen, in a memoir 

 " On the Mammals and Winter Birds of East Florida," &c. 

 (Bulletin of the Museum of Comparative Zoology at Harvard 

 College, Cambridge, Mass., vol. ii. No. 3.) R. M.] 



