ecalls the "theory of
the repetition of parts," of which so much use was made by the German
transcendentalists, such as Goethe,[369] Oken, Meckel and K. G. Carus, as
well as by Duges.

The third, and naturally the most important, ingredient in the _General
Morphology_ was the doctrine of evolution, in the form given to it by
Darwin. We have here no concern with Haeckel's evolutionary philosophy,
with the way in which he combined his evolutionism and his materialism
to form a queer Monism of his own. We are interested only in the way he
applied evolution to morphology, what modifications he introduced into
the principles of the science, and in general in what way he interpreted
the facts and theories of morphology in the light of the new knowledge.

We find that he repeats very much what Darwin said, giving, of course,
more detail to the exposition, and elaborating, particularly in his
recapitulation theory or "biogenetic law," certain doctrines not
explicitly stated by Darwin.

Like Darwin he held that the natural system is in reality genealogical.
"There exists," he writes, "one single connected natural system of
organisms, and this single natural system is the expression of real
relations which actually exist between all organisms, alike those now in
being on the earth and those that have existed there in some past time.
The real relations which unite all living and extinct organisms in one
or other of the principal groups of the natural system, are
genealogical: their relationship in form is blood-relationship; the
natural system is accordingly the genealogical tree of organisms, or
their genealogema.... All organisms are in the last resort descendants
of autogenous Monera, evolved as a consequence of the divergence of
characters through natural selection. The different subordinate groups
of the natural system, the categories of the class, order, family,
genus, etc., are larger or smaller branches of the genealogical tree,
and the degree of their divergence indicates the degree of genealogical
affinity of the related organisms with one another and with the common
ancestral form" (ii., p. 420).

The degree of systematic relationship is thus the degree of genealogical
affinity. It follows that the natural system of classification may be
converted straightway into a genealogical tree, and this is actually
what Haeckel does in the _General Morphology_. The genealogical trees
depicted in the second volume (plates i.-v