on size, colour, or the element inhabited; but naturalists have long
felt a profound conviction that there is a natural system. This system,
it is now generally admitted, must be, as far as possible, genealogical
in arrangement, that is, the co-descendants of the same form must be
kept together in one group, apart from the co-descendants of any other
form; but if the parent-forms are related, so will be their
descendants, and the two groups together will form a larger group. The
amount of difference between the several groups that is the amount of
modification which each has undergone is expressed by such terms as
genera, families, orders, and classes. As we have no record of the
lines of descent, the pedigree can be discovered only by observing the
degrees of resemblance between the beings which are to be classed. For
this object numerous points of resemblance are of much more importance
than the amount of similarity or dissimilarity in a few points. If two
languages were found to resemble each other in a multitude of words and
points of construction, they would be universally recognised as having
sprung from a common source, notwithstanding that they differed greatly
in some few words or points of construction. But with organic beings
the points of resemblance must not consist of adaptations to similar
habits of life: two animals may, for instance, have had their whole
frames modified for living in the water, and yet they will not be
brought any nearer to each other in the natural system. Hence we can
see how it is that resemblances in several unimportant structures, in
useless and rudimentary organs, or not now functionally active, or in
an embryological condition, are by far the most serviceable for
classification; for they can hardly be due to adaptations within a late
period; and thus they reveal the old lines of descent or of true
affinity.
We can further see why a great amount of modification in some one
character ought not to lead us to separate widely any two organisms. A
part which already differs much from the same part in other allied
forms has already, according to the theory of evolution, varied much;
consequently it would (as long as the organism remained exposed to the
same exciting conditions) be liable to further variations of the same
kind; and these, if beneficial, would be preserved, and thus be
continually augmented. In many cases the continued development of a
part, for instance, of the beak of a bird, or of the teeth of a mammal,
would not aid the species in gaining its food, or for any other object;
but with man we can see no definite limit to the continued development
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