GEOGRAPHICAL DISTRIBUTION. 27 



Professor Allen has thus been largely quoted from for the reason that 

 his views embody the identical opinions held and expressed by the natural- 

 ists of this expedition, and are peculiarly adapted to the circumstances of 

 this survey ; and, although the above remarks apply more particularly to 

 mammals, a law of generalization may be deduced, which will apply to 

 birds also, and probably we may eventually be able to include reptiles and 

 insects. 



As an addition to the views of Professor Allen, it may be mentioned 

 that, so far as regards color-variation, our attention should be drawn to the 

 singular mimicry of color as seen more particularly by the collectors of the 

 expedition in the Campestrian Region, or in localities contiguous thereto. 

 This, as has been mentioned in a different portion of this report, was 

 extremely well marked in the cases of reptiles, and such observations were 

 taken as to prove the case beyond the cavil of a doubt. In the Pkryno- 

 somce was it particularly noticeable, as also in some of the more brilliantly- 

 colored serpents ; and there can be no doubt that a law may yet be formu- 

 lated in this respect which will equally apply to all classes of animals. 



With regard to the distribution of birds and their color-variation, the 

 same general rule may be applied as to mammals, differing slightly, how- 

 ever, in some minor particulars, and perhaps further quotations from Pro- 

 fessor Allen* may serve to elucidate the matter better than any description 

 of my own. We must premise, however, by stating that, within the last 

 few years, so much has our knowledge increased respecting these interesting 

 members of the animal kingdom, " that the opinions formerly current 

 respecting the rank of a certain class of forms heretofore generally regarded 

 as specific have been radically modified. Intergradation has been fre- 

 quently traced between widely different forms ; a gradual coalescence in 

 scores of instances having been positively established, and rendered 

 extremely probable in a large number of others. 



"In North America, a geographical variation exhibits two marked phases 

 (as has already been stated), a differentiation with differences of latitude 

 and elevation ; and, secondly, differentiation with differences of longitude, 

 which, for convenience, may be termed respectively latitudinal and longi- 



* Proceedings of the Boston Society of Natural History, vol. xv, June, 1872. 



