On these same principles we see
how it is that the mutual affinities of the forms within each class are so
complex and circuitous. We see why certain characters are far more
serviceable than others for classification; why adaptive characters, though
of paramount importance to the beings, are of hardly any importance in
classification; why characters derived from rudimentary parts, though of no
service to the beings, are often of high classificatory value; and why
embryological characters are often the most valuable of all. The real
affinities of all organic beings, in contradistinction to their adaptive
resemblances, are due to inheritance or community of descent. The Natural
System is a genealogical arrangement, with the acquired grades of
difference, marked by the terms, varieties, species, genera, families,
etc.; and we have to discover the lines of descent by the most permanent
characters, whatever they may be, and of however slight vital importance.
The similar framework of bones in the hand of a man, wing of a bat, fin of
the porpoise, and leg of the horse--the same number of vertebrae forming
the neck of the giraffe and of the elephant--and innumerable other such
facts, at once explain themselves on the theory of descent with slow and
slight successive modifications. The similarity of pattern in the wing and
in the leg of a bat, though used for such different purpose--in the jaws
and legs of a crab--in the petals, stamens, and pistils of a flower, is
likewise, to a large extent, intelligible on the view of the gradual
modification of parts or organs, which were aboriginally alike in an early
progenitor in each of these classes.
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