ccasionally numbers of
vertebrae alter, that parts become soldered, that parts are lost, as tail
and toes, but we know <that?> here we can see that possibly a walking organ
might <?> be converted into swimming or into a gliding organ and so on to a
flying organ. But such gradual changes would not alter the unity of type
in their descendants, as parts lost and soldered and vertebrae. But we
can see that if this carried to extreme, unity lost,--Plesiosaurus. Here
we have seen the same organ is formed <?> <for> different purposes
<ten words illegible>: and if, in several orders of vertebrata, we could
trace origin <of> spinous processes and monstrosities &c. we should say,
instead of there existing a unity of type, morphology{154}, as we do
when we trace the head as being the vertebrae metamorphosed. Be it
observed that Naturalists, as they use terms of affinity without
attaching real meaning, here also they are obliged to use metamorphosis,
without meaning that any parent of crustacean was really an animal with
as many legs as crustacean has jaws. The theory of descent at once
explains these wonderful facts.

{152} This is, I believe, the first place in which the author uses
the words "theory of descent."

{153} The sentence should probably run, "Let us take the case of
the vertebrata: if we assume them to be descended from one parent,
then by this theory they have been altered &c."

{154} That is "we should call it a morphological fact."

Now few of the physiologists who use this language really suppose that
the parent of insect with the metamorphosed jaw, was an insect with
[more] so many legs, or that the parent of flowering plants, originally
had no stamens, or pistils or petals, but some other means of
propagation,--and so in other cases. Now according to our theory during
the infinite number of changes, we might expect that an organ used for a
purpose might be used for a different one by his descendant, as must
have been the case by our theory with the bat, porpoise, horse, &c.,
which are descended from one parent. And if it so chanced that traces of
the former use and structure of the part should be retained, which is
manifestly possible if not probable, then we should have the organs, on
which morphology is founded and which instead of being metaphorical
becomes plain and <and instead of being> utterly unintelligible becomes
simple matter of fact{155}.

{155} In the _Origin_