282 FIRST PRINCIPLES OF NATURAL CLASSIFICATION. 
characters, however, are found so uniformly prevalent 
through the species of a natural group, that they are 
mostly employed in defining genera and sub-genera : 
slight alterations from such standards nevertheless occur 
in species; and they consequently claim the especial 
attention of the ornithologist. That he may be con- 
vinced of this, let him compare the form, size, and 
manner of disposition of the scales upon the feet of a 
few different groups ; as, for example, a crow, shrike, 
chatterer, and flycatcher ; and he will at once perceive 
how singularly they are diversified. So, likewise, are the 
scales of reptiles and of fish. On proceeding to insects, 
we find this character rising in importance, particularly 
in the coleopterous order. Sometimes the wing-cases 
are punctured, either irregularly or in lines ; in others, 
they are either grooved, ribbed, wrinkled, spined, or 
tuberculated ; and each of these are again diversified in 
an almost infinity of ways. Better distinctions for 
species than these, cannot possibly be obtained ; and they 
accordingly are used for this purpose by the best 
writers. The same diversity may be observed both among 
the univalve and the bivalve shells ; the surface of which 
are diversified — according to the species — either 
with spines, nodules, tubercles, wrinkles, grooves, 
ridges, simple lines, or punctured lines ; and sometimes 
these lines cross each other like the fabric of a basket, 
and they are then termed cancellated. The sculpture 
of corals, and of the spines upon the different sea eggs 
(Echini), afford excellent specific distinctions, and may 
always be soemployed. Lastly, we may notice, under 
this head, that partial clothing on the surface of many 
animals, otherwise naked, which is termed pubescence. 
This is chiefly confined to insects ; and consists of downy 
hairs, of different degrees of density and length, spread 
either wholly or partially over their body. The highest 
developement of this is seen in the field bees (Bombus) ; 
but it is very prevalent among beetles, particularly such 
as represent, in their different circles, the order of 
HTymenoptera. This pubescence is either partial or 
