Chap. XIII, 
MOKPHOLOGY. ^ 
433 
under one species; we use descent in classing acknow¬ 
ledged varieties^ however different they may be from their 
parent; and I believe this element of descent is the hid¬ 
den bond of connexion which naturalists have sought 
under the term of the Natural System. On this idea of 
the natural system being, in so far as it has been perfected, 
genealogical in its arrangement, with the grades of differ¬ 
ence between the descendants from a common parent, 
expressed by the terms genera, families, orders, &c., we 
can understand the rules which we are compelled to 
follow in our classification. We can understand why we 
value certain resemblances far more than others; why 
we are permitted to use rudimentary and useless organs, 
or others of trifling physiological importance; why, in 
comparing one group with a distinct group, we summarily 
reject analogical or adaptive characters, and yet use these 
same characters within the limits of the same group. 
We can clearly see how it is that all living and extinct 
forms can be grouped together in one great system; 
and how the several members of each class are con¬ 
nected together by the most complex and radiating 
lines of affinities. We shall never, probably, disen¬ 
tangle the inextricable web of affinities between the 
members of any one class; but when we have a dis¬ 
tinct object in view, and do not look to some unknown 
plan of creation, we may hope to make sure but slow 
progress. 
Morphology,—W q have seen that the members of 
the same class, independently of their habits of life, 
resemble each other in the general plan of their organ¬ 
isation. This resemblance is often expressed by the 
term unity of type or by saying that the several 
parts and organs in the different species of the class 
are homologous. The whole subject is included under 
u 
