37§ 



Correspondence. I " t 



devoted to the 'Colors of North American Birds' there is much that is 

 suggestive and worthy of commendation, mixed with a great deal that is 

 weak and unphilosophical," etc. (p. 191). Or again: "While there is 

 much that is valuable in the work, and many points that are well taken, 

 Part II especially is largely vitiated by unsound reasoning, by misap- 

 prehension of facts, or by lack of general information on special points" 

 (p. 194). The trouble is that Mr. Keeler seems unable to distinguish 

 between pure speculation and reasonable hypothesis. 



3. It is not claimed that there is never any change in the color of 

 feathers without a moult, aside from the fading and very pronounced 

 change we know to take place simply by exposure of the plumage to the 

 elements. The case of the Oriole simply typifies a large class of cases 

 where there is a transitional, immature dress characteristic for a season 

 or two, according to the species, of the young male in a great many kinds 

 of birds. The evidence, not altogether negative, that this is what it 

 seems, and is generally believed to be, namely, a true transition stage, 

 where often it is difficult to find two birds marked exactly alike, is so 

 overwhelming and conclusive that the onus probandi fairly rests on the 

 supporters of the opposing theory that the birds are gradually acquiring 

 the perfect or adult plumage by a radical, gradual change of color in 

 the mature feather -without moult. To recite the evidence against this 

 kind of change would require far more space than can here be spared. 1 

 In this connection, however, it may be noted that a microscopical 

 examination of the mature feathers of Orioles, which Mr. Keeler assumes 

 gradually change from olive to black, will probably show that pigment 

 has very little to do with the case. Should such prove to be the fact the 

 question could be readily settled ; for it seems too much to suppose that 

 there can be sufficient structural or molecular change in the mature 

 feather to produce a radical change of color. 



4. This is a postulate I am surprised to see emanate from Mr. Wallace, or 

 even Mr. Keeler! It is true that we know little of the method of physio- 

 logical action resulting from climatic influences, but the results of this 

 potent force, encountered on every hand, are too evident to be overlooked. 

 That humidity, or its absence, acts directly on the fully formed feather 

 so as to cause the '■'■deposit,'' or "withdrawal," of pigment is a conception 

 too absurd for serious consideration, beyond the obvious fact that 

 feathers do fade through exposure, in the living bird as well as in the 

 museum specimen, somewhat in ratio to the degree of aridity and the 

 intensity of the bleaching sunlight to which they are exposed. But the 

 gradual evolution of a permanent change of color, such as marks geo- 

 graphical races or representative species for example, must obviously be 

 due to the long-continued action of the environing conditions upon the 



1 See some remarks on this point, however, in Bull. Am. Mus. Nat. Hist., Vol. 

 V, p. 108. 



