ON THE ART OF CLASSIFYING. 27 
accustom themselves to strict order in all their other branches of 
study. 
The foundation of all Zoological division is the Species. By this 
is understood the assemblage of all the individuals which have more 
conformity to each other, than to other similar creatures; which, by 
means of mutual impregnation, can generate prolific individuals and 
propagate themselves by generation, so that it can be inferred from 
analogy that they all sprung from a single pair. By specific cha- 
racter is understood the collection of all the characteristics which 
are shewn to be permanent. Those characteristics, on the other 
hand, by which different individuals of a species vary amongst 
themselves, and which are attributable to deviation from species, are 
called varieties. 
The causes of varieties consist in the influence of external cir- 
cumstances, and in the mixture of other similar species. Differences 
from this last cause are called Hybrids. The form is here a com- 
bination of the two parents. 
Such Hybrids appear to be limited, fabulous stories apart, to 
those species which have great mutual resemblance. They are in 
general not prolific—not able to continue their race. They occur 
therefore, beyond doubt, in a state of nature extremely rarely, and 
are rather the consequence of the constrained state of servitude in 
which our domestic animals exist. This cause therefore is not of a 
kind to disturb the regular course of nature and to endanger the pre- 
servation of the species. On the other hand, varieties produced by 
the influence of external circumstances, by climate, difference of food 
and mode of life, are able to engender young that are prolific. But 
they do not suggest any doubt that we ought, perchance, to receive 
them for species. It must moreover be remarked, that those varieties 
of ordinary species which on account of the pliancy of their organi- 
sation and their tenaciousness of life are able to live in every climate, 
and appear, for the most part, to have followed man over the entire 
surface of the earth, are the most striking and the most numerous. 
By Genus is understood a second group formed by the union of 
like species, as the species was formed by that of like individuals, 
Species which in general have a striking resemblance in their orga- 
nisation, form a genus. The idea of genus is so natural that we 
meet with traces of it even in the language of children. Still all 
genera are not natural. Many of them have been formed upon some 
