Characters as Adaptive and Specific. 177 



even if it were practicable on the present occa- 

 sion — to adduce this evidence in detail, since an 

 exceedingly good sample of it may be found in 

 a small book which is specially devoted to consider- 

 ing the question in its relation to birds. I allude 

 to an essay by Mr. Charles Dixon, entitled Evolu- 

 tion without Natural Selection (1885). In this work 

 Mr. Dixon embodies the results of five years' " care- 

 ful working at the geographical distribution and 

 variations of plumage of Palaearctic birds and their 

 allies in various other parts of the world" ; and 

 shows, by a large accumulation of facts, not only 

 that there is no utility to be suggested in reference 

 to the minute or trivial differences of colouration 

 which he describes ; but also that these differences 

 are usually correlated with isolation on the one 

 hand, or with slight differences of climate on the 

 other. Now it will be shown later on that both 

 these agents can be proved, b}'' independent evidence, 

 capable of inducing changes of specific type with- 

 out reference to utility : therefore the correlation 

 which Mr. Dixon unquestionably establishes between 

 apparently useless (because utterly trivial) specific 

 distinctions on the one hand, and isolation or 

 climatic change on the other, constitutes additional 

 evidence to show that the uselessness is not only 

 apparent, but real. Moreover I have collected a 

 number of cases where such minute differences of 

 colour between allied species of birds happen to 

 affect parts of the plumage which are concealed — as 

 for instance, the breast and abdomen of creepers. In 

 such cases it seems impossible to suggest how natural 

 selection can have operated, seeing that the parts 

 11. N 



