276 FIRST PRINCIPLES OF NATURAL CLASSIFICATION. 
that a warm temperature, upon some animals, produces 
the same effect as cold does upon others, whose natural 
metropolis is in warm latitudes. This variation in 
size will often occur in individuals found in the same 
country, or even on the same spot ; just as we observe, 
among a family of children, different degrees of height 
and of robustness. This variation, however, is more 
observable among insects than among quadrupeds, while 
in birds it is somewhat rare. We do not here include 
domesticated races as examples, because it is well known 
that the several breeds of horses, pigs, cattle, fowls, &c: 
not only vary in size in a most remarkable manner, but 
assume, in a state of domestication, such different mo- 
difications of their usual characters, that, were we to 
discover them in a wild state, they would be viewed as 
distinct species. The most variable species of birds, in 
regard to their size, are the hangnests of America ; but 
more especially those of the genus Cassicus — the largest 
of which, the elegant crested cassican*, varies almost 
in every district it inhabits: and yet it is still doubt- 
ful whether a better acquaintance with some of these 
supposed varieties, particularly those of the red-rumped 
species }, may not make known peculiarities of habits 
and of manners, which may justify us in considering 
them distinct species. Generally speaking, however, 
there must be something more than a mere difference 
in size, to authorise our making it the only ground of 
specific difference. 
(338.) Shape, or contour, is the second property 
of form: there are scarcely any instances in which 
animals, possessing a peculiarity of shape, however 
slight it may be, are not distinct species from their 
congeners. A peculiarity in the shape of the wing- 
feathers, or of the bill in birds; in the direction of 
the horns of oxen, antelopes, and beetles; in the 
shape of the antenne, or of its joints, in almost all 
insects ; and many other peculiarities which will readily 
suggest themselves; may all be taken as good and 
* Cassicus cristatus, Ornithological Drawings, pl. 32. 
+ Cass. hemorrhous and affinis, Ornithological Drawings, pl. 1, 2. 
