r type into a higher type--ever
occurred. To use his own words:

"If, however, all Cephalous Mollusca, _i.e._, all Cephalopoda,
Gasteropoda, and Lamellibranchiata, be only modifications by
excess or defect of the parts of a definite archetype, then, I
think, it follows as a necessary consequence, that no
anamorphosis takes place in this group. There is no progression
from a lower to a higher type, but merely a more or less complete
evolution of one type. It may indeed be a matter of very grave
consideration whether true anamorphosis ever occurs in the whole
animal kingdom. If it do, then the doctrine that every natural
group is organised after a definite archetype, a doctrine which
seems to me as important for zooelogy as the theory of definite
proportions for chemistry, must be given up."

It is of great historical interest to notice how closely actual
consideration of the facts of the animal kingdom took zooelogists to an
idea of evolution, and yet how far they were from it as we hold it
now. It is fashionable at the present time to attempt to depreciate
the immense change introduced by Darwin into zooelogical speculation,
and the method employed is largely partial quotation, or reference to
the kind of ideas found in papers such as this memoir by Huxley. The
comparison between the types of the great groups and the combining
proportions of the chemical elements shows clearly that Huxley
regarded the structural plans of the great groups as properties
necessary and inherent in these groups, just as the property of a
chemical element to combine with another chemical substance only in a
fixed proportion is necessary and inherent in the existing conception
of it. There was no glimmer of the idea that these types were not
inherent, but merely historical results of a long and slow series of
changes produced by the interaction of the varied conditions of life
and the intrinsic qualities of living material.

In two lectures delivered at the Royal Institution in 1854 and 1855,
the one on "The Common Plan of Animal Forms," the other on "The
Zooelogical Arguments Adduced in Favour of the Progressive Development
of Animal Life in Time," show, so far as the published abstracts go,
the same condition of mind. The idea of progressive development of all
life from common forms was not unknown to Huxley and his
contemporaries, but was rejected by them. In the first of these