30 
MEMOIR OF 
bears the greatest resemblance, either naturally, 
or agreeably to the principles of his system ; and 
having fixed on it, he denominates the variety a 
species. Thus, the hooded crow is a species of 
crow. So also, having determined which des- 
cription of the humming bird he will consider 
generic, he denominates the varieties and devia- 
<< tions from this description within certain limits, 
as the different species of humming birds. To 
each of the different species some name is given 
descriptive of its chief distinctions, and thus, 
those who agree in the use of the same classifica- 
tion, and of the same names of the different 
species, can readily enable each other to find in 
books any particular species. 
Thus, genera and species are founded in re- 
semblances ; and by assorting animals together, 
agreeably to any particular resemblances which 
may be determined on for the purpose, whether 
toes, teeth, claws, beaks, habitation, food, habits, 
&c. a system is formed, and the particular point 
of resemblance adopted by any individual natu- 
ralist distinguishes his particular system, and in 
the superior advantages for scientific purposes of 
any author’s classification, consists the superiority 
of his system. 
The most obvious resemblances are not, how- 
ever, always to be chosen for the purposes of 
system. 
“ The rudest wanderer in the fields, “ observes 
an eminent writer,”* may imagine that the pro- 
* Dr Brown’s Philosophy of the Human Mind. 
