CHAPTER XIV. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY -- EMBRYOLOGY -- RUDIMENTARY ORGANS.
1. CLASSIFICATION. (continued)
With species in a state of nature, every naturalist has in fact brought
descent into his classification; for he includes in his lowest grade, that
of species, the two sexes; and how enormously these sometimes differ in the
most important characters is known to every naturalist: scarcely a single
fact can be predicated in common of the adult males and hermaphrodites of
certain cirripedes, and yet no one dreams of separating them. As soon as
the three Orchidean forms, Monachanthus, Myanthus, and Catasetum, which had
previously been ranked as three distinct genera, were known to be sometimes
produced on the same plant, they were immediately considered as varieties;
and now I have been able to show that they are the male, female, and
hermaphrodite forms of the same species. The naturalist includes as one
species the various larval stages of the same individual, however much they
may differ from each other and from the adult; as well as the so-called
alternate generations of Steenstrup, which can only in a technical sense be
considered as the same individual. He includes monsters and varieties, not
from their partial resemblance to the parent-form, but because they are
descended from it.
As descent has universally been used in classing together the individuals
of the same species, though the males and females and larvae are sometimes
extremely different; and as it has been used in classing varieties which
have undergone a certain, and sometimes a considerable amount of
modification, may not this same element of descent have been unconsciously
used in grouping species under genera, and genera under higher groups, all
under the so-called natural system? I believe it has been unconsciously
used; and thus only can I understand the several rules and guides which
have been followed by our best systematists. As we have no written
pedigrees, we are forced to trace community of descent by resemblances of
any kind. Therefore, we choose those characters which are the least likely
to have been modified, in relation to the conditions of life to which each
species has been recently exposed. Rudimentary structures on this view are
as good as, or even sometimes better than other parts of the organisation.
We care not how trifling a character may be--let it be the mere inflection
of the angle of the jaw, the manner in which an insect's wing is folded,
whether the skin be covered by hair or feathers--if it prevail throughout
many and different species, especially those having very different habits
of life, it assumes high value; for we can account for its presence in so
many forms with such different habits, only by inheritance 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,
concur 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; and we know that such aggregated
characters have especial value in classification.
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