. The
resemblance of the greyhound and racehorse is hardly more fanciful than the
analogies which have been drawn by some authors between very distinct
animals. On my view of characters being of real importance for
classification, only in so far as they reveal descent, we can clearly
understand why analogical or adaptive character, although of the utmost
importance to the welfare of the being, are almost valueless to the
systematist. For animals, belonging to two most distinct lines of descent,
may readily become adapted to similar conditions, and thus assume a close
external resemblance; but such resemblances will not reveal--will rather
tend to conceal their blood-relationship to their proper lines of descent.
We can also understand the apparent paradox, that the very same characters
are analogical when one class or order is compared with another, but give
true affinities when the members of the same class or order are compared
one with another: thus the shape of the body and fin-like limbs are only
analogical when whales are compared with fishes, being adaptations in both
classes for swimming through the water; but the shape of the body and
fin-like limbs serve as characters exhibiting true affinity between the
several members of the whale family; for these cetaceans agree in so many
characters, great and small, that we cannot doubt that they have inherited
their general shape of body and structure of limbs from a common ancestor.
So it is with fishes.

As members of distinct classes have often been adapted by successive slight
modifications to live under nearly similar circumstances,--to inhabit for
instance {428} the three elements of land, air, and water,--we can perhaps
understand how it is that a numerical parallelism has sometimes been
observed between the sub-groups in distinct classes. A naturalist, struck
by a parallelism of this nature in any one class, by arbitrarily raising or
sinking the value of the groups in other classes (and all our experience
shows that this valuation has hitherto been arbitrary), could easily extend
the parallelism over a wide range; and thus the septenary, quinary,
quaternary, and ternary classifications have probably arisen.

As the modified descendants of dominant species, belonging to the larger
genera, tend to inherit the advantages, which made the groups to which they
belong large and their parents dominant, they are almost sure to spread
widely, and to seize on more and more