278 FIRST FRINCIPLES OF NATURAL CLASSIFICATION. 
tions to the foregoing rule that at present occur to us ; 
they relate almost as much to the size as to the shape 
of an object ; but peculiarities drawn from the latter 
circumstance are much more to be depended upon than 
those of the former. 
(339.) A form is modified by difference in its pro- 
portions. Hence, the comparative shortness or longness 
of an animal, or of its parts, when compared to another 
which it closely resembles, is (with the exceptions last 
mentioned) usually a sign that it is a distinct species. 
The comparative length of the tail in quadrupeds and 
birds, and of the wings in birds and insects, are excel- 
lent specific distinctions: the antenne of insects do not 
supply, in all cases, so good a criterion ; for they often 
vary considerably, both as to structure and length, in 
the sexes of the same species: the same uncertainty 
attends the employment of specific characters drawn 
from their legs. In birds and quadrupeds, however, 
the structure of these latter members afford distinctions 
for groups; and these latter modifications give usa cer- 
tain index for the determination of species. The 
entomologist should pay particular attention to the pro- 
portions of two insects, which come so close to each 
other, that he may be in doubt as to their specific dif- 
ference: the greater enlargement of the thorax, the 
wings, the feet, or even of the antenne, may frequently 
indicate a real difference. 
(340.) The most general distinction of species is 
manifested by their colours: among these, however, we 
must not include black ; for not only are nearly all the 
species of entire families (as the Harpalide MacL.) and 
sub-families (doling Sw.) of this colour, but nearly 
the whole of the saprophagous beetles are of the same 
sombre hue. Colours, among quadrupeds, unless when 
domesticated, seldom vary in individuals of the same 
species ; and still more rarely among birds, when arrived 
at maturity. It should be remembered, however, that 
in the feathered creation, young birds are almost always 
clothed in the garb of their mother ; and that among the 
