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THE AMEBIC AN NATURALIST [Vol. XLIX 



question of the fixity of the subspecies of mammals and 

 birds lias, to my knowedge, never before been put to ex- 

 perimental test. 15 



Now, in view of the subject matter of the present paper, 

 it would ill become me to underrate the value of such 

 tests. But I think that I have had enough to do with the 

 experimental method in zoology to make me realize its 

 rigid limitations. It is seldom indeed that we are able 

 to perform a really crucial experiment and to obtain un- 

 equivocal results. Moreover, the mills of the gods grind 

 slowly, while the single human life is short. 



I am therefore disposed to attach considerable impor- 

 tance to what have been called "Nature's experiments." 

 Certain of these have been cited by Grinnell and Swarth 16 

 in their bearing on the subspecies question. For ex- 

 ample, two well-marked local races or subspecies of song- 

 sparrow occur in southern California, on opposite sides 

 of the high mountain range which divides the coastal 

 plain from the desert interior. These races are sepa- 

 rated nearly everywhere by the mountain barrier, but at 

 certain points passes through the latter occur, permitting 

 of in juration from one side to the other. 



Now these authors find localities in which the coast 

 form of song-sparrow has penetrated, for considerable 

 distances, into the desert and has become established 

 there. Nevertheless, these invaders, which have perhaps 

 been exposed for many generations to the desert at- 

 mosphere, have retained the darker pigmentation and 

 other characteristics proper to the coastal plain. 



Again, both Grinnell and Taylor 17 have taken speci- 

 mens of Peromyscus maniculatus, which they believe to be 

 typical representatives of the desert race sonoriensis, 



G. S. Miller, who finds that this transplanted race is^merely the unmodified 



in the British Museum, London, 1912.) 



i« University of California Publications in Zoology, Vol. 10, No. 9, 1913. 

 "University of California Publications in Zoology, Vol. 7, No. 7, 1911. 



