third
in the viscera. He shewed how the widely different groups of cephalous
molluscs could be conceived as modifications of this structure, and
extended the conception so as to cover all other molluscs.

Quite apart from the anatomical value of this paper, and although all
technical details have been omitted here, it is necessary to say that
merely as a series of intricate anatomical descriptions and
comparisons, this memoir was one of the most valuable of any that
Huxley wrote. The working out of the theory of the archetype is
peculiarly interesting to compare with modern conceptions. To those of
us who began biological work after the idea of evolution had been
impressed upon anatomical work, it is very difficult to follow
Huxley's papers without reading into them evolutionary ideas. In the
article upon Mollusca, written for the ninth edition of the
_Encyclopaedia Britannica_, by Professor Ray Lankester, the same device
of an archetypal or, as Lankester calls it, a schematic mollusc, is
employed in order to explain the relations of the different structures
found in different groups of molluscs to one another. Lankester's
schematic mollusc differs from Huxley's archetypal mollusc only as a
finished modern piece of mechanism, the final result of years of
experiment, differs from the original invention. The method of
comparing the schematic mollusc with the different divergent forms in
different groups is identical, and yet, while the ideas of Darwin are
accepted in every line of Lankester's work, Huxley was writing six
years before the publication of _The Origin of Species_. There was
growing up in Huxley's mind, partly from his own attempts to arrange
the anatomical facts he discovered in an intelligible series, the idea
that within a group the divergencies of structure to be found had come
about by the modification of an original type. Not only did he
conceive of such an evolution as the only possible explanation of the
facts, but he definitely used the word _evolution_ to convey his
ideas. On the other hand, he was firmly convinced that such evolution
was confined within the great groups. For each group there was a
typical structure, and modifications by defect or excess of the parts
of the definite archetype gave rise to the different members of the
group. Moreover, he confined this evolution in the strictest possible
way to each group; he did not believe that what was called
anamorphosis--the transition of a lowe