Not only was the prevailing method of investigation faulty, but actual
knowledge of a large part of the animal kingdom was extremely limited.
In the minds of most zooelogists the animal kingdom was divided into
two great groups: the vertebrates and invertebrates. The vertebrate,
or back-boned, animals were well known; comparatively speaking they
are all built upon the type of man; and human anatomists, who indeed
made up the greater number of all anatomists, using their exact
knowledge of the human body, had studied many other vertebrates with
minute care, and, from man to fishes, had arranged living vertebrates
very much in the modern order. But the invertebrates were a vague and
ill-assorted heap of animals. It was not recognised that among them
there were many series of different grades of ascending complexity,
and there was no well-known form to serve as a standard of comparison
for all the others in the fashion that the body of man served as a
standard of comparison for all vertebrates. Here and there, a few
salient types such as insects and snails had been picked out, but
knowledge of them helped but little with a great many of the
invertebrates. The great Linnaeus had divided the animal kingdom into
four groups of vertebrates: mammals, birds, reptiles, and fishes, but
for the invertebrates he had done no more than to pick out the insects
as one group and to call everything else "Vermes" or worms.
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