s convenient to use his term for it--the biogenetic law
(_Biogenetische Grundgesetz_)--to distinguish it from the laws of
Meckel-Serres and von Baer, with which it is so often confused.

Haeckel's statement of it may best be summarised in his own words,
"Ontogeny, or the development of the organic individual, being the
series of form-changes which each individual organism traverses during
the whole time of its individual existence, is immediately conditioned
by phylogeny, or the development of the organic stock (phylon) to which
it belongs.

"Ontogeny is the short and rapid recapitulation of phylogeny,
conditioned by the physiological functions of heredity (reproduction)
and adaptation (nutrition). The organic individual (as a morphological
individual of the first to the sixth order) repeats during the rapid and
short course of its individual development the most important of the
form-changes which its ancestors traversed during the long and slow
course of their palaeontological evolution according to the laws of
heredity and adaptation.

"The complete and accurate repetition of phyletic by biontic development
is obliterated and abbreviated by secondary contraction, as ontogeny
strikes out for itself an ever straighter course; accordingly, the
repetition is the more complete the longer the series of young stages
successively passed through.

"The complete and, accurate repetition of phyletic by biontic
development is falsified and altered by secondary adaptation, in that
the bion[372] during its individual development adapts itself to new
conditions: accordingly the repetition is the more accurate the greater
the resemblance between the conditions of existence under which
respectively the bion and its ancestors developed" (ii., p. 300).

The last two propositions, it will be observed, are taken over almost
verbally from F. Mueller.

Now we have seen that the natural system of classification gives a true
picture of the genealogical relationships of organisms, that the smaller
and larger classificatory groups correspond to greater or lesser
branches of the genealogical tree. If ontogeny is a recapitulation of
phylogeny, we must expect to find the embryo repeating the organisation
first of the ancestor of the phylum, then of the ancestor of the class,
the order, the family and the genus to which it belongs. There must be a
threefold parallelism between the natural system, ontogeny and phylogeny
(ii., pp. 421-2).