REVIEWS. 369 
urements demonstrate what he claims— that many or most species, 
vary in total stature, in the length of different members, and in 
the relative proportions of different parts, from twelve to eighteen 
per cent. of the meandimensions. This, it should be remembered, 
is independent of geographical differentiation ; and it will, we trust, 
be enough to put us on our guard, against too ready acceptance of 
slight discrepancies in size as an element in our diagnoses of spe- 
cies. To cite but a single case in point :— twenty-seven speci- 
mens of Parus atricapillus, taken in the same locality and at nearly 
the same time, differ over an inch in length; that is, they grade 
between the extremes of P. septentrionalis and P. Carolinensis. 
‘* Climatic differentiation,” like individual variation, is shown to 
occur under three principal phases: — in total size, in length and 
stoutness of bill, and in color. The first of these is well known 
and need not detain us. The second consists in the curious fact 
above mentioned, of development of the bill in inverse ratio to the 
size of the bird, with decrease in latitude. Climatic variation in 
color, Mr. Allen holds, may be in respect of both latitudinal and 
longitudinal conditions. In the matter of latitude, he shows a 
gradual increase in intensity of color to the southward, probably 
dependent upon the greater energy of the sunlight ; and in this way 
disposes of several West Indian forms, commonly reputed as spe- 
cies, pointing out that they are not more different from their Flori- 
dan analogues, than these are from New England examples. As 
to longitudinal variation, Mr. Allen brings prominently forward 
the fact that ‘‘the general tendency from the East westward, is 
to darker or deeper colors in specimens of the same species” (p. 
237), and supports this by numerous unquestionable cases, many of 
which will readily occur to the reader. The very notable excep- 
tions afforded by the bleached specimens of the Colorado desert, 
furnish the occasion for what we regard as decidedly the most im- 
portant point in the whole discussion, and one that we do not re- 
member to have seen in print before. Under head of ‘ Causes of 
Climatic Variation” (p. 239), Mr. Allen takes humidity of the at- 
mosphere, as determined by the mean annual rain-fall, to be the 
real cause of this intensity of color. In examining Dr. Foster’s 
“ The Mississippi Valley,” some time since, we were struck with 
the determinations there made of the hygrometric influences result- 
ing in the production of forest, prairie and desert, according to 
mean annual water-supply, and, at the same time, perceived the 
