l with ours, separated from them by anatomical features,
but so united with them by form and external features that many among
them have been at first associated together.

This is what Cuvier has called subordination of characters,
distinguishing between characters that control the organization and
those that are not essentially connected with it. The skill of the
naturalist consists in detecting the difference between the two, so
that he may not take the more superficial features as the basis of his
classification, instead of those important ones which, though often less
easily recognized, are more deeply rooted in the organization. It is a
difference of the same nature as that between affinity and analogy, to
which I have alluded before, when speaking of the ingrafting of certain
features of one type upon animals of another type, thus producing a
superficial resemblance, not truly characteristic. In the Reptiles, for
instance, there are two groups,--those devoid of scales, with naked
skin, laying numerous eggs, but hatching their young in an imperfect
state, and the Scaly Reptiles, which lay comparatively few eggs, but
whose young, when hatched, are completely developed, and undergo no
subsequent metamorphosis. Yet, notwithstanding this difference in
essential features of structure, and in the mode of reproduction and
development, there is such an external resemblance between certain
animals belonging to the two groups that they were associated together
even by so eminent a naturalist as Linnaeus. Compare, for instance, the
Serpents among the Scaly Reptiles with the Caecilians among the Naked
Reptiles. They have the same elongated form, and are both destitute
of limbs; the head in both is on a level with the body, without any
contraction behind it, such as marks the neck in the higher Reptiles,
and moves only by the action of the back-bone; they are singularly alike
in their external features, but the young of the Serpent are hatched in
a mature condition, while the young of the type to which the Caecilians
belong undergo a succession of metamorphoses before attaining to a
resemblance to the parent. Or compare the Lizard and the Salamander, in
which the likeness is perhaps even more striking; for any inexperienced
observer would mistake one for the other. Both are superior to the
Serpents and Caecilians, for in them the head moves freely on the neck
and they creep on short imperfect legs. But the Lizard is clothed wit