ation is by far the most important, and most of the
others are in a sense merely special cases of this fundamental law. To
this law of differentiation is due the increase in complexity or
perfection of organisation which is shown by all the animal series.
Bronn himself recognised the great similarity of this law of progressive
differentiation to Milne-Edwards' principle of the division of labour;
he seems, however, to have arrived at it independently.

Bronn's third factor in the production of variety of form is adaptation
to environment, or better, functional response to environment. Bronn
gives an excellent account of adaptational modifications and calls
attention, just as Milne-Edwards did, to the numerous analogies of
structure which adaptation brings about. He works out the interesting
view that there is some connection between classificatory groups and
adaptational forms, especially such as are connected with the function
of locomotion:--"Based upon a common characteristic method of locomotion
are whole or nearly whole sub-phyla (Hexapoda), classes (mammals and
reptiles, birds, fishes, gastropods, pteropods, brachiopods, Bryozoa,
Rotifera, jelly-fish, polypes, sponges), sub-classes (mobile and
immobile lamellibranchs, echinoderms, walking and swimming Crustacea,
parasitic and free-living worms, and so on), often, however, only orders
and quite small groups (snakes, eels, bats, sepias, medusae, etc.)" (p.
141).

It was characteristic of the 'forties and 'fifties that transcendental
anatomy, along with Nature-philosophy, went rather out of fashion, its
false simplicities and premature generalisations being overwhelmed by
the flood of new discoveries. A few stalwarts indeed upheld
transcendental views. We have already discussed the morphological system
built up by Richard Owen in the late 'forties, a system transcendental
in its main lines. We have seen the vertebral theory of the skull still
maintained in the 'fifties by such men as Reichert and Koelliker, and we
find J. V. Carus in 1853[314] taking it as almost conclusively proved.[315]

We may mention, too, as showing clear marks of the influence of
transcendental ideas, L. Agassiz's work on the principles of
classification.[316] And Serres, who was Geoffroy's chief disciple,
recanted not a whit of his doctrine of recapitulation, but re-affirmed
and expanded it from time to time, and particularly in a lengthy memoir
published in 1860.[317] But in general we may