Van Beneden, Gegenbauer, and others, and so gradually arrived at a conception of what he called the “archetype” of the cephalous molluscs.  As the word archetype was borrowed from old metaphysical ideas dating back to the time of Plato, he took care to state that what he meant by it was no more than a form embodying all that could be affirmed equally respecting every single kind of cephalous mollusc, and by no means an “idea” upon which it could be supposed that animal forms had been modelled.  He described this archetype, and showed the condition of the different systems of organs which it could be supposed to possess, and how these organs were modified in the different existing groups.  This archetypal mollusc of Huxley’s was a creature with a bilaterally symmetrical head and body.  On the ventral side of the body it possessed a peculiar locomotor appendage, the so-called foot, and the dorsal surface of the body secreted a shell.  Its nervous system consisted of three pairs of ganglia or brains, one pair in the head, one in the foot, and a 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