144 PROBLEMS OF GENETICS 



There is evidence that in Grenada, as in St. Vincent, the black is 

 gradually ousting the original type, but the process has not gone 

 so far as in St. Vincent. Austin Clark very properly compares 

 this case of the Sugar-birds with that of Papilio turnus, which as 

 is well-known, has a black female in the southern parts of its dis- 

 tribution, in addition to a female of the yellow type, but in the 

 Northern States the black female does not occur. 



During the present year P. R. Lowe, who lately studied 

 Coerebas on a large scale in the West Indies, has published an 

 important paper on the subject. 29 He calls attention to the 

 fact that Cory recently found a black form of Coereba on Los 

 Roques Islands, and he himself discovered another on the 

 Testigos Islands. Both localities are on the coast of Venezuela, 

 far from St. Vincent and Grenada. The whole problem is thus 

 further complicated by the fact that the black varieties have, 

 as we are almost driven to admit, arisen independently in remote 

 places. Improbable as this conclusion may be, it is still more 

 difficult to regard all the black forms as derived from one source. 

 For first, they present definite small differences from each other; 

 and secondly we have to remember a consideration of greater 

 importance, that the very fact that each island has its own type 

 must be accepted as proving that the localities are effectively 

 isolated from each other, and that migration must be a very 

 rare event. 



The rarity of such illustrative cases is, I believe, more ap- 

 parent than real. It is probably due to the extreme reluctance 

 of systematists to admit that such things can be, and of course 

 to the almost complete absence of knowledge as to the genetic 

 behaviour of wild animals and plants. Only in such examples 

 as this of the Coereba, where colour constitutes the sole differ- 

 ence, or that of the moths which have been minutely studied by 

 many collectors, does the significance of the facts appear. The 

 arrangement of catalogues and collections is such that much 

 practical difficulty of a quite unnecessary kind is introduced. For 

 example, in this very case of Coereba, I find the British Museum 

 has a fine series from Grenada including 3 normals and 1 1 black, 



29 Ibis, 191 2, pp. 523-8. 



