eritance from a common parent. We may err in this respect in regard to
single points of structure, but when several characters, let them be ever
so trifling, occur together throughout a large group of beings having
different habits, we may feel almost sure, on the theory of descent, that
these characters have been inherited from a common ancestor. {426} And we
know that such correlated or aggregated characters have especial value in
classification.

We can understand why a species or a group of species may depart, in
several of its most important characteristics, 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 community 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 existence--are generally the most
constant, we attach especial 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 think, clearly
see why embryological characters are of such high classificatory
importance. Geographical distribution may sometimes be brought usefully
into play in classing large and widely-distributed 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 important distinction between
real affinities and analogical or adaptive resemblances. Lamarck first
called attention 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 {427} instances: thus
Linnaeus, misled by external appearances, actually classed an homopterous
insect as a moth. We see something of the same kind even in our domestic
varieties, as in the thickened stems of the common and swedish turnip