Hilaire has strongly insisted on the high importance of relative position or connexion in homologous parts; they may differ to almost any extent in form and size, and yet remain connected together in the same invariable order.

We never find, for instance, the bones of the arm and forearm, or of the thigh and leg, transposed.

Hence the same names can be given to the homologous bones in widely different animals.

We see the same great law in the construction of the mouths of insects: what can be more different than the immensely long spiral proboscis of a sphinx-moth, the curious folded one of a bee or bug, and the great jaws of a beetle? Yet all these organs, serving for such widely different purposes, are formed by infinitely numerous modifications of an upper lip, mandibles, and two pairs of maxillae.

The same law governs the construction of the mouths and limbs of crustaceans.