Henshaw on the Hermit Thrush. 137 



tremes of size of the three forms, as indicated by my measvii'emeuts, 

 here follow : — 



Comparing the smallest specimens of jyallasi and auduhoni with 

 the largest individual of nanus, a considerable percentage of the 

 former forms will be found to fall below the latter in size. Obvi- 

 ously, however, this would not be a fair comparison, since it is highly 

 exceptional that this extreme of size is attained by nanus ; in fact, 

 but one specimen in my series does so, the measurements of those 

 next it falling considerably short. Attention may here be called 

 also to the extremely small size of the smallest individual of /w/^«s^', 

 as shown by the above figures. No other of the series compares 

 with it in this respect, the next in size having wings of 3.43, and 

 upwards, with the other dimensions corresponding. Doubtless it 

 was just such another individual which fell into Audubon's hands, 

 and furnished occasion for his name nanus. 



Similar comparison between the largest jmllasi and smallest au- 

 duboni gives a similar result. But again, the largest and smallest 

 individuals of these birds respectively represent comparatively rare 

 exceptions. 



It appears, therefore, that, while between unusual extremes of 

 the three forms intergradation actually does take place, it is far 

 frorri being " complete," and that, in fact, it is no greater than is 

 usual in the cases of other species, with their varieties, in which 

 change of geographical limits has been accompanied by increased or 

 diminished size. Our tables further demonstrate that the greater 

 number of individuals are actually identifiable by the test of size 

 alone. 



If by the recognition of varieties nothing were to be gained but 

 the opportunity of increasing the list of named birds, not only 

 would no harm result from ignoring them, but, on the contrary, 

 positive benefit. Such, however, is very far from being the case. 

 Varieties, — species in embryo, — if understood to be the result of 

 natural laws, the expression of new conditions under which species 



