It will be observed that there is here implied an analogy between the
biogenetic law and the law of von Baer, for both assert that development
proceeds from the general to the special, that the farther back in
development you go the more generalised do you find the structure of the
embryo; both assert, too, that differentiation of structure takes place
not in one progressive or regressive line, but in several diverging
directions.

But the analogy between the biogenetic law and the Meckel-Serres law is
even more obvious, and the resemblance between the two is much more
fundamental. It is a significant fact that in his theory of the
threefold parallelism Haeckel merely resuscitated in an evolutionary
form a doctrine widely discussed in the 'forties and 'fifties,[373] and
championed particularly by L. Agassiz,[374] a doctrine which must be
regarded as a development or expansion of the Meckel-Serres law.[375] It
is the view that a parallelism exists between the natural system,
embryonic development, and palaeontological succession. Actually, as
Agassiz stated it, the doctrine applied neither to types, nor as a
general rule to classes, but merely to orders. It was well exemplified,
he thought, in Crinoids:--"The successive stages of the embryonic growth
of Crinoids typify, as it were, the principal forms of Crinoids which
characterise the successive geological formations. First, it recalls the
Cistoids of the palaeozoic rocks, which are represented in its simple
spheroidal head; next the few-plated Platycrinoids of the Carboniferous
period; next the Pentacrinoids of the Lias and Oolite with their whorls
of cirrhi; and finally, when freed from its stem, it stands as the
highest Crinoid, as the prominent type of the family in the present
period" (p. 171).

The Meckel-Serres law, it will be remembered, expressed the idea that
the higher animals repeat in their ontogeny the adult organisation of
animals lower in the scale. Since Haeckel recognised clearly that a
linear arrangement of the animal kingdom was a mere perversion of
reality, and that a branching arrangement of groups more truly
represented the real relations of animals to one another, he could not
of course entertain the Meckel-Serres theory in its original form. But
he accepted the main tenet of it when he asserted that each stage of
ontogeny had its counterpart in an adult ancestral form. Such ancestral
forms might or might not be in existence as real species at t