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 t, 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 antennae, 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, pi. 32. 



t Caw. hcemorrhous and affinis, Ornithological Drawings, pL 1, 2. 



