1907] Beebe: Geographic Variation in Birds. 37 



Theoretically, and to the evolutionist in the widest sense 

 of the word, (for whose use nomenclature, to all intents and pur- 

 poses, solely exists), the results of experimental biology are of 

 greatest importance, but the forms thus artificially produced 

 should no more be added to the lists of natural species than 

 should the brahmas and plymouth rock fowls of our barnyards. 

 As in so many scientific questions there is a border-land difficult 

 to define, and in this case it consists of the forms transplanted 

 by man in more or less distant times, which have developed new 

 characteristics (as in the Porto Santo rabbits). In my opinion, 

 none of these should be placed on our lists without being stigma- 

 tized as to the human influence involved. 



To those of us who are primarily students of the broader 

 aspects of evolution, the results of experimental biology appear 

 most valuable in giving clearer insight into the conception of the 

 ideal and only natural classification — one arranged in the three 

 planes of space. 



G — Correlation with Organic Selection. 



Any correlation of the results outlined in the present paper 

 with the various theories of evolution must be tentative in the 

 present state of our knowledge. Keeping this in mind, the fol- 

 lowing explanation appears among the most logical and probable. 



The results of the effects of humidity are open to the follow- 

 ing interpretations: (1), they may reflect a reversal of evolu- 

 tional variation along the line of least resistance, from inca 

 through dialeucos to ridgwayi and beyond; (2), they may indi- 

 cate an advance in evolution, paralleling the condition found in 

 wild tropical types; (3), they may represent merely indefinite 

 fortuitous variation, brought about by a change in the environ- 

 ment. Whichever we choose for the present as the most reason- 

 able working hypothesis, it seems to me that we have an excellent 

 argument in support of the theory of organic selection. 



We know that melanin is a non-ferrugineous non-hemato- 

 genous intrinsic pigment, absorbing the violet rays which, at 

 least in ourselves, are particularly irritating to the skin. If the 

 same phenomenon holds true of birds, an increase of melanin in 

 the epidermal structures would certainly prove of advantage in 

 a hot moist climate, and might thus be considered an adaptive 

 structural response. 



In the case of Scardafella we see an instance of remarkably 

 rapid physiological reaction to a radical change of the environ- 

 ment. In a state of nature such a change might be imagined as 



