, Ed. i. p. 438, vi. p. 602, the author,
referring to the expressions used by naturalists in regard to
morphology and metamorphosis, says "On my view these terms may be
used literally."

<_Embryology._> This general unity of type in great groups of organisms
(including of course these morphological cases) displays itself in a
most striking manner in the stages through which the foetus passes{156}.
In early stage, the wing of bat, hoof, hand, paddle are not to be
distinguished. At a still earlier <stage> there is no difference between
fish, bird, &c. &c. and mammal. It is not that they cannot be
distinguished, but the arteries{157} <illegible>. It is not true that
one passes through the form of a lower group, though no doubt fish more
nearly related to foetal state{158}.

{156} See _Origin_, Ed. i. p. 439, vi. p. 605.

{157} In the _Origin_, Ed. i. p. 440, vi. p. 606, the author argues
that the "loop-like course of the arteries" in the vertebrate
embryo has no direct relation to the conditions of existence.

{158} The following passages are written across the page:--"They
pass through the same phases, but some, generally called the higher
groups, are further metamorphosed.

? Degradation and complication? no tendency to perfection.

? Justly argued against Lamarck?"

This similarity at the earliest stage is remarkably shown in the course
of the arteries which become greatly altered, as foetus advances in life
and assumes the widely different course and number which characterize
full-grown fish and mammals. How wonderful that in egg, in water or air,
or in womb of mother, artery{159} should run in same course.

{159} An almost identical passage occurs in the _Origin_, Ed. i. p.
440, vi. p. 606.

Light can be thrown on this by our theory. The structure of each
organism is chiefly adapted to the sustension of its life, when
full-grown, when it has to feed itself and propagate{160}. The structure
of a kitten is quite in secondary degree adapted to its habits, whilst
fed by its mother's milk and prey. Hence variation in the structure of
the full-grown species will _chiefly_ determine the preservation of a
species now become ill-suited to its habitat, or rather with a better
place opened to it in the economy of Nature. It would not matter to the
full-grown cat whether in its young state it was more or less eminently
feline, so that it become so whe