CHAPTER XIV. MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY -- EMBRYOLOGY -- RUDIMENTARY ORGANS.
We have seen that the members of the same class, independently of their
habits of life, resemble each other in the general plan of their
organisation. This resemblance is often expressed by the term "unity of
type;" or by saying that the several parts and organs in the different
species of the class are homologous. The whole subject is included under
the general term of Morphology. This is one of the most interesting
departments of natural history, and may almost be said to be its very soul.
What can be more curious than that the hand of a man, formed for grasping,
that of a mole for digging, the leg of the horse, the paddle of the
porpoise, and the wing of the bat, should all be constructed on the same
pattern, and should include similar bones, in the same relative positions?
How curious it is, to give a subordinate though striking instance, that the
hind feet of the kangaroo, which are so well fitted for bounding over the
open plains--those of the climbing, leaf-eating koala, equally well fitted
for grasping the branches of trees--those of the ground-dwelling, insect or
root-eating, bandicoots--and those of some other Australian marsupials--
should all be constructed on the same extraordinary type, namely with the
bones of the second and third digits extremely slender and enveloped within
the same skin, so that they appear like a single toe furnished with two
claws. Notwithstanding this similarity of pattern, it is obvious that the
hind feet of these several animals are used for as widely different
purposes as it is possible to conceive. The case is rendered all the more
striking by the American opossums, which follow nearly the same habits of
life as some of their Australian relatives, having feet constructed on the
ordinary plan. Professor Flower, from whom these statements are taken,
remarks in conclusion: "We may call this conformity to type, without
getting much nearer to an explanation of the phenomenon;" and he then adds
"but is it not powerfully suggestive of true relationship, of inheritance
from a common ancestor?"