Hen SHAW on the Hermit Thrush 
137 
tremes of size of the three forms, as indicated by my measurements, 
here follow : — 
pallasi 
(largest), 
Wing, 
3.85. 
Tail, 
2 . 87 . 
Bill, . 51 . 
Tarsus, 
1 . 14 . 
« 
(smallest), 
a 
3 . 30 . 
a 
2 . 50 . 
tt 
' • 
u 
1 . 05 . 
auduboni (largest), 
a 
4 . 25 . 
it 
3 . 35 . 
“ . 56 . 
it 
1 . 16 . 
a~ 
(smallest), 
a 
3 . 70 . 
It 
2 . 97 . 
“ . 54 . 
it 
1 . 13 . 
nanus 
(largest), 
u 
3 . 67 . 
U 
3 . 00 . 
“ . 50 . 
a 
1 . 16 . 
a 
(smallest), 
U 
3 . 25 . 
a 
2 . 55 . 
“ . 49 . 
« 
1 . 04 . 
Comparing the smallest specimens of pallasi and auduboni 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 pallasi , 
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 pallasi 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 
from 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 
