uish the embryos only by their size. In my possession
are two little embryos in spirit, whose names I have omitted to attach,
and at present I am quite unable to say to what class they belong. They
may be lizards or small birds, or very young mammalia, so complete is
the similarity in the mode of formation of the head and trunk in these
animals. The extremities, however, are still absent in these embryos.
But even if they had existed in the earliest stage of their development
we should learn nothing, for the feet of lizards and mammals, the wings
and feet of birds, no less than the hands and feet of man, all arise
from the same fundamental form." 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. No one supposes that the stripes on the whelp of
a lion, or the spots on the young blackbird, are of any use to these
animals.

The case, however, is different when an animal, during any part of its
embryonic career, is active, and ha