e
evolution of the race. Just as he kept clear of the notion of the scale
of beings, so he avoided the snare of the Meckel-Serres theory of
recapitulation, according to which the embryo of the highest animal,
man, during its development climbs the ladder upon the rungs of which
the whole animal series is distributed, in its gradual progression from
simplicity to complexity. The law of development which he adopts is that
of von Baer, which states that development is essentially
differentiation, and that as a result embryos belonging to the same
group resemble one another the more the less advanced they are in
development. There can be little doubt that he was indebted to von Baer
for the idea, and in the later editions of the _Origin_ he acknowledges
this by quoting the well-known passage in which von Baer tells how he
had two embryos in spirit which he was unable to refer definitely to
their proper class among Vertebrates.[354]

Not only are embryos more alike than adults, because less
differentiated, but it is in points not directly connected with the
conditions of existence, not strictly adaptive, that their resemblance
is strongest (p. 440)--think, for instance, of the arrangement of aortic
arches common to all vertebrate embryos. Larval forms are to some extent
exceptions to this rule, for they are often specially adapted to their
particular mode of life, and convergence of structure may accordingly
result. All these facts require an explanation. "How, then, can we
explain these several facts in embryology--namely, the very general, but
not universal, difference in structure between the embryo and the
adult--of parts in the same individual embryo, which ultimately become
very unlike and serve for different purposes, being at this early period
of growth alike--of embryos of different species within the same class,
generally but not universally, resembling each other--of the structure
of the embryo not being closely related to its conditions of existence,
except when the embryo becomes at any period of life active and has to
provide for itself--of the embryo apparently having sometimes a higher
organisation than the mature animal, into which it is developed" (pp.
442-3). Obviously all these facts are formally explained by the doctrine
of descent. But Darwin goes further, he tries to show exactly how it is
that the embryos resemble one another more than the adults. He thinks
that the phenomenon results from two princip