iii.) are nothing more than
graphic representations of the ordinary systematic relationships of
organisms, with a few hypothetical ancestral groups or forms thrown in
to give the whole a genealogical turn.

If the genealogical tree is truly represented by the natural system, it
would seem that for each genus a single ancestral form must be
postulated, for each group of genera a single more primitive form, and
so in general for each of the higher classificatory categories, right up
to the phylum. Species of one genus must be descended from a generic
ancestral form, genera of one family from a single family _Urform_, and
so on for the higher categories.

This consequence was explicitly recognised by Haeckel. "Genera and
families," he writes, "as the next highest systematic grades, are
extinct species which have resolved themselves into a divergent bunch of
forms (_Formenbueschel_)" (ii., p. 420).

The archetype of the genus, family, order, class and phylum was thus
conceived to have had at some past time a real existence.

The natural system of classification is based upon a proper appreciation
of the distinction between homological and analogical characters.
Haeckel, following Darwin, naturally interprets the former as due to
inheritance, the latter as due to adaptation, using these words, we may
note, in their accepted meaning and not in the abstract empty sense he
had previously attributed to them.[370] Similarly the "type of
organisation," in von Baer's sense, was due to heredity, the "grade of
differentiation" to adaptation.

So far Haeckel merely emphasised what Darwin had already said in the
_Origin of Species_. But by his statement of the "biogenetic law," and
particularly by the clever use he made of it, Haeckel went a step beyond
Darwin, and exercised perhaps a more direct influence upon evolutionary
morphology than Darwin himself.

Haeckel was not the original discoverer of the law of recapitulation. It
happened that a few years before the publication of Haeckel's _General
Morphology_, a German doctor, Fritz Mueller by name, stationed in Brazil,
had been working on the development of Crustacea under the direct
inspiration of Darwin's theory, and had published in 1864 a book[371] in
which he showed that individual development gave a clue to ancestral
history.

He conceived that progressive evolution might take place in two
different ways. "Descendants ... reach a new goal, either by deviating
sooner or