OVERLAPPING FORMS 155 



is complicated by the undoubted effects of an uncertain amount 

 of migration, and in many, perhaps all, districts, the winter 

 population differs from the summer population of the same 

 localities. The existence of these seasonal ebbs and flows is 

 now well known to ornithologists, and most of the bird species 

 of temperate regions are subject to them. 



Difficult as it may be to conceive the actual process of origin 

 of the two types auratus and cafer, it is I think still harder to 

 suggest any possible circumstance which can have determined 

 their development as distinct races, or which can maintain 

 that distinctness when created. Some will no doubt be disposed 

 to appeal once more to our ignorance and suggest that if we only 

 knew more we should see that the yellow quills, the black 

 * 'moustache" and the red crescent, specially qualify auratus 

 for the north and eastern region, and the red quills, red "mou- 

 stache" and absence of crescent fit cafer to the conditions of 

 its homes. Each can judge for himself, but my own view is 

 that this is a vain delusion, and that to cherish it merely blunts 

 the receptivity of the mind, which if unoccupied with such fancies 

 would be more ready to perceive the truth when at last it shall 

 appear. Think of the range of conditions prevailing in the 

 country occupied by auratus — a triangle with its apex in Florida 

 and its base the whole Arctic region of North America. Is it 

 seriously suggested that there is some element common to the 

 "conditions" of such an area which demands a nuchal crescent 

 in the Flickers, though the birds of the cafer area, almost equally 

 varied, can dispense with the same character? Curiously enough, 

 the geographical variation of Sphyropicus varius, another though 

 a very different Woodpecker 6 shows that conversely the nuchal 

 crescent can be dispensed with in the Eastern form though it 

 is assumed by the Western. 7 



6 The Sap-suckers feed on trees and somewhat resemble our Spotted Wood- 

 peckers in general appearance. Colaptes feeds on the ground and corresponds 

 perhaps rather with the European Green Woodpecker. 



7 For an introduction to this example I am indebted to Mr. W. D. Miller of 

 the American Museum of Natural History. Some account of the facts is given by 

 Baird, Brewer, and Ridgway (A Hist, of N. Amer. Birds, 1874, II, pp. 540, 544, 

 etc.). S. varius occupies the whole country in suitable places from the Atlantic 

 to the eastern slopes of the Rockies, and all Mexico to Guatemala. 5. nuchalis was 



