" The larvae of most crustaceans, at corresponding stages of
development, closely resemble each other, however different the adults may
become; and so it is with very many other animals. A trace of the law of
embryonic resemblance occasionally lasts till a rather late age: thus
birds of the same genus, and of allied genera, often resemble each other in
their immature plumage; as we see in the spotted feathers in the young of
the thrush group. In the cat tribe, most of the species when adult are
striped or spotted in lines; and stripes or spots can be plainly
distinguished in the whelp of the lion and the puma. We occasionally,
though rarely, see something of the same kind in plants; thus the first
leaves of the ulex or furze, and the first leaves of the phyllodineous
acacias, are pinnate or divided like the ordinary leaves of the
leguminosae.
The points of structure, in which the embryos of widely different animals
within the same class resemble each other, often have no direct relation to
their conditions of existence. We cannot, for instance, suppose that in
the embryos of the vertebrata the peculiar loop-like courses of the
arteries near the branchial slits are related to similar conditions--in the
young mammal which is nourished in the womb of its mother, in the egg of
the bird which is hatched in a nest, and in the spawn of a frog under
water. We have no more reason to believe in such a relation than we have
to believe that the similar bones in the hand of a man, wing of a bat, and
fin of a porpoise, are related to similar conditions of life.
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