onic development
of two animals keep identical the more nearly the two animals are
related, when Huxley wrote, was founded on a much smaller number of
facts than now are known. Since 1860 an enormous bulk of embryological
investigation has been published, and the total result has been to
confirm Huxley's position in the fullest possible way. A certain
number of exceptions have been found, but these exceptions are so
obviously special adaptations to special circumstances that their
existence only makes the general truth of the proposition more clear.
The most common kind of exception occurs when two closely related
animals live under very different conditions. For instance, many
marine animals have close allies that in comparatively recent times
have taken to live in fresh water. The conditions of life in fresh
water are very different, especially for delicate creatures
susceptible to rapid changes of temperature, or unable to withstand
strong currents. Thus most of the allies of the fresh-water crayfish,
which live in the sea, lay eggs from which there are soon hatched
minute, almost transparent larvae, exceedingly unlike the adult. In the
comparatively equable temperature of sea-water, and in the usual
absence of strong currents, these small larvae, as Huxley shewed later
in his volume on the _Crayfish_, live a free life, obtaining their own
food, and by a series of slow transformations gradually acquire the
adult form. In fresh water, however, the delicate larvae would be
unable to live, and the mode of development is different. The series
of slow transformations is condensed, and takes place almost entirely
inside the egg-shell; so that, when hatching occurs, the young
crayfish is exceedingly like the adult. Apart from such special cases,
it is true that the study of development affords a clear test of
closeness of structural affinity.

Huxley then proceeds to discuss the development of man.

"Is he something apart? Does he originate in a totally different
way from dog, bird, frog, and fish, thus justifying those who
assert him to have no place in nature, and no real affinity with
the lower world of animal life? Or does he originate in a similar
germ, pass through the same slow and gradually progressive
modifications, depend on the same contrivances for protection and
nutrition, and finally enter the world by the help of the same
mechanism? The reply is not doubtful for a mom