, embryology enters a strong protest. It is notorious
that the wings of birds and bats, and the legs of horses or other
quadrupeds, are undistinguishable at an early embryonic period, and
that they become differentiated by insensibly fine steps. Embryological
resemblances of all kinds can be accounted for, as we shall hereafter
see, by the progenitors of our existing species having varied after
early youth, and having transmitted their newly-acquired characters to
their offspring, at a corresponding age. The embryo is thus left almost
unaffected, and serves as a record of the past condition of the species.
Hence it is that existing species during the early stages of their
development so often resemble ancient and extinct forms belonging to the
same class. On this view of the meaning of embryological resemblances,
and indeed on any view, it is incredible that an animal should have
undergone such momentous and abrupt transformations as those above
indicated, and yet should not bear even a trace in its embryonic
condition of any sudden modification, every detail in its structure
being developed by insensibly fine steps.

He who believes that some ancient form was transformed suddenly through
an internal force or tendency into, for instance, one furnished with
wings, will be almost compelled to assume, in opposition to all analogy,
that many individuals varied simultaneously. It cannot be denied that
such abrupt and great changes of structure are widely different from
those which most species apparently have undergone. He will further be
compelled to believe that many structures beautifully adapted to all the
other parts of the same creature and to the surrounding conditions,
have been suddenly produced; and of such complex and wonderful
co-adaptations, he will not be able to assign a shadow of an
explanation. He will be forced to admit that these great and sudden
transformations have left no trace of their action on the embryo.
To admit all this is, as it seems to me, to enter into the realms of
miracle, and to leave those of science.

CHAPTER VIII. INSTINCT.

Instincts comparable with habits, but different in their
origin--Instincts graduated--Aphides and ants--Instincts
variable--Domestic instincts, their origin--Natural instincts of
the cuckoo, molothrus, ostrich, and parasitic bees--Slave-making
ants--Hive-bee, its cell-making instinct--Changes of instinct and
structure not necessarily simultaneous--Diff