CHARACTERS OF SPECIES. — SCULETURE. 981 
deceptive: see them alive, and they look like drops of 
burnished gold ; look at them in cabinets, and they appear 
like different insects, entirely of a dull yellowish colour : 
yet let these very specimens be plunged in warm water, 
and, while the moisture continues, all their rich metallic 
hues will revive, as if by magic. The colours of many 
of the apterous insects, especially of the spiders, would 
enable us to discriminate the species with much pre- 
cision ; but unfortunately they are more evanescent than 
those of any other insects, nor has any method been yet 
discovered for preserving them. We are therefore 
obliged, reluctantly, to seek for other means to define 
the species. The same remark applies to a large pro- 
portion of the crabs ; although they will exhibit, if well 
preserved, some faint indication of their original hues. 
Changes of colour, in individuals of the same species of 
birds, will be effected by extreme age : the green plumage 
of parrots changes to yellow ; and aged females of the 
gallinaceous order are known to assume the plumage and 
colours of the male bird. We know not whether this 
circumstance has been observed in wild birds ; but many 
instances of this change are upon record in the cases of 
pheasants, fowls, and other domesticated species. 
(342.) Sculpture is the last distinction of species 
which we shall here enumerate. This term compre- 
hends all those various modes by which nature has 
diversified the naked parts of birds, the elytre or ex- 
ternal wings and other parts of insects, the scales of 
reptiles and of fish, and the hard or testaceous covering 
of molluscous animals. The bodies of quadrupeds and 
birds, being covered either with hair or feathers, show 
very little of this peculiarity: the horns of the ruminating 
animals, however, present us with different kinds of sur- 
faces, as shown in the rings and nodules upon them ; 
these are greatly diversified, and almost invariably in- 
dicate a difference in species. Sculpture, in birds, is 
restricted to grooves upon the bill of certain species 
of hornbills and toucans ; and to the form, and mode 
of division, of the scales on the feet. These latter 
