156 PROBLEMS OF GENETICS 



Allen points out the interesting additional fact that super- 

 posed upon each of the two distinct forms, auratus and cafer, 

 are many geographical variations which can very naturally be 

 regarded as climatic. Each decreases in size from the North 

 southward, as so many species do. 8 They become paler in the 

 arid plains, and show the ordinary phases which are seen in 

 other birds having the same distribution. Such differences 

 we may well suppose to be determined directly or indirectly, by 

 environment, and we may anticipate with fuller knowledge it 

 will be possible to distinguish variations of this nature as in the 

 broad sense environmental, from the larger differences separating 

 the two main types of Colaptes, which I surmise are altogether 

 independent of such influences. 



It is generally supposed that phenomena like those now so 

 well established in the case of Colaptes are very exceptional, and 

 as has already been stated a number of circumstances must 

 combine in order that they may be produced. I suspect however 

 that the examples are more numerous than is commonly thought. 

 In all likelihood the three forms Sphyropicus varius, nuchalis 

 and ruber are in a very similar condition though the details 

 have not, so far as I know, been worked out. A complex example 

 which is closely parallel to the case of Colaptes was described by 

 F M. Chapman 9 at the same date as Allen's work. This is the 

 case of Quiscalus, the Crackles, which in the North American 

 Continent have three fairly distinct forms which Chapman speaks 

 of as Q. aeneus, Q. quiscula, and Q. quiscula aglaeus. The birds 

 are all, so far as pigment is concerned, dark blackish brown, but 

 the head and mantle have superposed a metallic sheen of inter- 

 ference-colours which in the various forms take different tints, 



first known from the Southern Rockies only, but many were afterwards taken in 

 Utah. 5. ruber is restricted to the Pacific coast. In Ridgway's opinion all three 

 are geographical forms of one species. In ruber the sexes are alike having both a 

 great extension of the red in the throat, and a red crescent. The male of nuchalis 

 grades to the ruber form, but the female does not. This female has some red in 

 the throat like the male of varius, whereas the female of varius has a whitish throat. 



8 Not only vertebrates but the marine Crustacea and Mollusca illustrate this 

 curious "principle" of variation, as Canon Norman formerly pointed out to me 

 with abundant illustrations. There are of course cases to the contrary also. 



9 Chapman, F. M., Bull. Amer. Mus., IV, 1892, p. i; see also Ridgway, Birds 

 of North and Middle America, 1902, Part II, p. 214. 



