orphosed leaves; and thus not only can the number,
position and transitional states of these several organs, but likewise
their monstrous changes, be most lucidly explained. It is believed that
the same laws hold good with the gemmiferous vesicles of Zoophytes. In
the same manner the number and position of the extraordinarily
complicated jaws and palpi of Crustacea and of insects, and likewise
their differences in the different groups, all become simple, on the
view of these parts, or rather legs and all metamorphosed appendages,
being metamorphosed legs. The skulls, again, of the Vertebrata are
composed of three metamorphosed vertebrae, and thus we can see a meaning
in the number and strange complication of the bony case of the brain. In
this latter instance, and in that of the jaws of the Crustacea, it is
only necessary to see a series taken from the different groups of each
class to admit the truth of these views. It is evident that when in each
species of a group its organs consist of some other part metamorphosed,
that there must also be a "unity of type" in such a group. And in the
cases as that above given in which the foot, hand, wing and paddle are
said to be constructed on a uniform type, if we could perceive in such
parts or organs traces of an apparent change from some other use or
function, we should strictly include such parts or organs in the
department of morphology: thus if we could trace in the limbs of the
Vertebrata, as we can in their ribs, traces of an apparent change from
being processes of the vertebrae, it would be said that in each species
of the Vertebrata the limbs were "metamorphosed spinal processes," and
that in all the species throughout the class the limbs displayed a
"unity of type{457}."

{456} See _Origin_, Ed. i. p. 436, vi. p. 599, where the parts of
the flower, the jaws and palpi of Crustaceans and the vertebrate
skull are given as examples.

{457} The author here brings _Unity of Type_ and _Morphology_
together.

These wonderful parts of the hoof, foot, hand, wing, paddle, both in
living and extinct animals, being all constructed on the same framework,
and again of the petals, stamina, germens, &c. being metamorphosed
leaves, can by the creationist be viewed only as ultimate facts and
incapable of explanation; whilst on our theory of descent these facts
all necessary follow: for by this theory all the beings of any one
class, say of the mammalia, are su