instance, to call certain groups of animals by one name, while Cuvier had
chosen to call them by another.

These divisions are, first, the most comprehensive groups, the primary
divisions, called branches by some, types by others, and divided by some
naturalists into so-called sub-types, meaning only a more limited
circumscription of the same kind of group; next we have classes, and these
also have been divided into sub-classes, then orders and sub-orders,
families, sub-families, and tribes; then genera, species, and varieties.
With reference to the question, whether these groups really exist in
Nature or are merely the expression of individual theories and opinions,
it is worth while to study the works of the early naturalists, in order to
trace the natural process by which scientific classification has been
reached; for in this, as in other departments of learning, practice has
always preceded theory. We do the thing before we understand why we do it:
speech precedes grammar, reason precedes logic; and so a division of
animals into groups, upon an instinctive perception of their differences,
has preceded all our scientific creeds and doctrines. Let us, therefore,
proceed to examine the meaning of these names as adopted by naturalists.

When Cuvier proposed his four primary divisions of the animal kingdom, he
added his argument for their adoption,--_because_, he said, they are
constructed on four different plans. All the progress in our science since
his time confirms this result; and I shall attempt to show that there are
really four, and only four, such structural ideas at the foundation of the
animal kingdom, and that all animals are included under one or another of
them. But it does not follow, that, because we have arrived at a sound
principle, we are therefore unerring in our practice. From ignorance we
may misplace animals, and include them under the wrong division. This is a
mistake, however, which a better insight into their organization