larva, will, it may safely be concluded, affect the structure of the
adult; in the same manner as any malconformation affecting the early
embryo, seriously affects the whole organisation of the adult. The
several parts of the body which are homologous, and which, at an early
embryonic period, are alike, seem liable to vary in an allied manner:
we see this in the right and left sides of the body varying in the same
manner; in the front and hind legs, and even in the jaws and limbs,
varying together, for the lower jaw is believed to be homologous with
the limbs. These tendencies, I do not doubt, may be mastered more or
less completely by natural selection: thus a family of stags once
existed with an antler only on one side; and if this had been of any
great use to the breed it might probably have been rendered permanent
by natural selection.
Homologous parts, as has been remarked by some authors, tend to cohere;
this is often seen in monstrous plants; and nothing is more common than
the union of homologous parts in normal structures, as the union of
the petals of the corolla into a tube. Hard parts seem to affect the
form of adjoining soft parts; it is believed by some authors that the
diversity in the shape of the pelvis in birds causes the remarkable
diversity in the shape of their kidneys. Others believe that the shape
of the pelvis in the human mother influences by pressure the shape of
the head of the child. In snakes, according to Schlegel, the shape of
the body and the manner of swallowing determine the position of several
of the most important viscera.
The nature of the bond of correlation is very frequently quite obscure.
M. Is. Geoffroy St. Hilaire has forcibly remarked, that certain
malconformations very frequently, and that others rarely coexist,
without our being able to assign any reason. What can be more singular
than the relation between blue eyes and deafness in cats, and the
tortoise-shell colour with the female sex; the feathered feet and skin
between the outer toes in pigeons, and the presence of more or less
down on the young birds when first hatched, with the future colour of
their plumage; or, again, the relation between the hair and teeth in
the naked Turkish dog, though here probably homology comes into play?
With respect to this latter case of correlation, I think it can hardly
be accidental, that if we pick out the two orders of mammalia which are
most abnormal in their dermal coverings, viz. Cetacea (whales) and
Edentata (armadilloes, scaly ant-eaters, etc.), that these are likewise
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