64 THE GROWTH OF GROUPS 



by migration or by the fact that its members were sterile 

 with all animals that were not of the colony. There is, 

 however, no reason why they should not become a 

 species in a certain sense, for if the whole ten had been 

 sent to a museum with a note to the effect that they 

 had been caught in such and such a district, they would 

 have been called a species, sub-species, variety, or race 

 according to choice, even though there were no others 

 like them in the world. 



Melanism is sometimes regarded as a peculiar pheno- 

 menon, as different from other heritable variation, but 

 it seems that the difference lies only in the fact that me- 

 lanism is very conspicuous in our eyes, hence it has 

 often been noticed. The following chapters will show 

 that other conspicuous colour characters occur in the 

 same way as melanism. 



Before leaving the subject of melanism I will quote 

 a passage from a paper by Messrs. Clarke and Hamilton 

 (The Zoologist, 1891), which has an interesting bearing 

 on the question. 



" The following note by the Rev. P. A. Keating of 

 Athlone appeared in the Field in 1893 : it gives a very 

 good account of a peculiar and as yet unexplained pheno- 

 menon in the history of the Irish black rat. 



" ' BLACK RAT IN COUNTY WEXFORD. 



' ' During a short visit to Co. Wexford early last 

 October I was informed that a large colony of black rats 

 suddenly put in an appearance in that county. I re- 

 paired with a friend to the locality with no small degree 

 of curiosity. We quietly entered a field of oaten stubble, 



