later whilst still on the way towards the form of their
parents, or by passing along this course without deviation, but then
instead of standing still advancing still farther" (Eng. trans., p.
111). In the former case the developmental history of descendants agrees
with that of the ancestors only up to a certain point and then diverges.
"In the second case the entire development of the progenitors is also
passed through by the descendants, and, therefore, so far as the
production of a species depends upon this second mode of progress, the
historical development of the species will be mirrored in its
developmental history" (p. 112).

Of course the recapitulation of ancestral history will be neither
literal nor extended. "The historical record preserved in developmental
history is gradually _effaced_ as the development strikes into a
constantly straighter course from the egg to the perfect animal, and it
is frequently _sophisticated_ by the struggle for existence which the
free-living larvae have to undergo" (p. 114).

It follows that "the primitive history of a species will be preserved in
its developmental history the more perfectly the longer the series of
young stages through which it passes by uniform steps; and the more
truly, the less the mode of life of the young departs from that of the
adults, and the less the peculiarities of the individual young states
can be conceived as transferred back from later ones in previous periods
of life, or as independently acquired" (p. 121).

Applying these principles to Crustacea, he concluded that the shrimp
_Peneus_ with its long direct development gave the best and truest
picture of the ancestral history of the Malacostraca, and that
accordingly the nauplius and the zoaea larvae represented important
ancestral stages. He conceived it possible so to link up the various
larval forms of Crustacea as to weave a picture of the primeval history
of the class, and he made a plucky attempt to work out the phylogeny of
the various groups.

The thought that development repeats evolution was already implicit in
the first edition of the _Origin_, but the credit for the first clear
and detailed exposition of it belongs to F. Mueller.

In much the same form as it was propounded by Mueller it was adopted by
Haeckel, and made the corner-stone of his evolutionary embryology.
Haeckel gave it more precise and more technical formulation, but added
nothing essentially new to the idea.

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