426 
CLASSIFICATION. 
Chap. XIII. 
And we know tliat such correlated or aggregated cha¬ 
racters have especial value in classification. 
We can understand why a species or a group of spe¬ 
cies may depart, in several of its most important cha¬ 
racteristics, from its allies, and yet be safely classed with 
them. This may be safely done, and is often done, as 
long as a sufficient number of characters, let them be 
ever so unimportant, betrays the hidden bond of com¬ 
munity of descent. Let two forms have not a single 
character in common, yet if these extreme forms are 
connected together by a chain of intermediate groups, 
we may at once infer their community of descent, and 
we put them all into the same class. As we find organs 
of high physiological importance—those which serve to 
preserve life under the most diverse conditions of exist¬ 
ence—are generally the most constant, we attach espe¬ 
cial value to them; but if these same organs, in another 
group or section of a group, are found to differ much, we 
at once value them less in our classification. We shall 
hereafter, I tliink, clearly see why embryological cha¬ 
racters are of such high classificatory importance. 
Geographical distribution may sometimes be brought 
usefully into play in classing large and widely-distri¬ 
buted genera, because all the species of the same genus, 
inhabiting any distinct and isolated region, have in all 
probability descended from the same parents. 
We can understand, on these views, the very im¬ 
portant distinction between real affinities and analogical 
or adaptive resemblances. Lamarck first called atten¬ 
tion to this distinction, and he has been ably followed 
by Macleay and others. The resemblance, in the shape 
of the body and in the fin-like anterior limbs, between 
the dugong, which is a pachydermatous animal, and the 
whale, and between both these mammals and fishes, is 
analogical. Amongst insects there are innumerable in- 
