278 PROCEEDINGS OF THE CANADIAN INSTITL'TE. 



any one can speak positively on the subject — and also without 

 heeding the fact that in other regions the distribution of this same 

 bird may be quite different, in accordance with different sur- 

 roundings." 



Mr. Thorn {)Son then proceeded to enumerate the better-known 

 elements of environment that tend to modify geographical distribu- 

 tion, and gave a large number of illustrations of these modifications. 



Temperature, as varied by latitude, elevation, slope, exposure, etc., 

 he considered of prime importance, and particularly dwelt on the 

 accuracy with which birds adhere in their summer habitat to the 

 exact isotherm that affords the nice balance of surroundings which is 

 most favorable to the species. 



Barometric pressure, rainfall, formation of surface, character of 

 soil, proximity of bodies of water and of currents, exposure to 

 . cei'tain winds, presence of competitors or of parasites, hygienic con- 

 ditions, and vegetation were all instanced and illustrated in their 

 bearing on distribution. The last, he stated, was of evident import- 

 ance, and probably ranked next to temperature, on which, however, 

 it is largely dependent. 



"These," he continued, " arc among the known factors of environ- 

 ment, but there is evidence that many other more obscure influences 

 are in o;)eration, and there can be little doubt that every force that is 

 or ever will be found in the whole range of the solar spectrum will 

 be ultimately proven an important element in the various problems 

 of distribution, so that the question filially resolves itself into a 

 serearch after the total environment of each species. 



I have been thus minute in tracing the known causes and aspects 

 of distribution in order to give a fair idea of the intricacy of the 

 problem presented, and I think it the more necessary because of late 

 I have been compelled to make a stand for the j)rinciples of work 

 herein maintained. I do not think any one will gainsay them ; I 

 have, at least, the best authorities of the day in harmony with my 

 views in general, if not in detail, and it is on these lines that I have 

 liad so often to condenni Canadian work. 



It will be seen that, since so much care and detail is required, no 



