h a character that it does not, destroy the
harmony of the whole."[54] We seize here the relation of the principle
of the adaptedness of parts to the problem of the variety of form. The
former is in a sense a regulative and conservative principle which
lays down limits beyond which variation may not stray. In itself it is
not a fountain of change; there must be another cause of change. This
thought is of great importance for theories of descent.

Cuvier has no theory to account for the variety of form: he contents
himself with a classification. There are two main ways of classifying
forms; you may classify according to single organs or according to the
totality of organs. By the first method you can have as many
classifications as you have organs, and the classifications will not
necessarily coincide. Thus you can divide animals according to their
organs of digestion into two classes, those in which the alimentary
canal is a sac with one opening (zoophytes) and those in which the
canal has two openings,[55] a curious forestalment, in the rough, of
the modern division of Metazoa into Coelentera and Coelomata.

It is only by taking single organs that you can arrange animals into
long series, and you will have as many series as you take organs. Only
in this way can you form any _Echelle des etres_ or graded series; and
you can get even this kind of gradation only within each of the big
groups formed on a common plan of structure; you can never grade, for
example, from Invertebrates to Vertebrates through intermediate
forms[56] (which is perfectly true, in spite of Amphioxus and
Balanoglossus!).

In the _Regne Animal_ Cuvier restricts the application of the idea of
the _Echelle_ within even narrower limits, refusing to admit its
validity within the bounds of the vertebrate phylum, or even within
the vertebrate classes. This seems, however, to refer to a seriation
of whole organisms and not of organs, so that the possibility of a
seriation of organs within a class is not denied. Cuvier was, above
all, a positive spirit, and he looked askance at all speculation which
went beyond the facts. "The pretended scale of beings," he wrote, "is
only an erroneous application to the totality of creation of partial
observations, which have validity only when confined to the sphere
within which they were made."[57] This remark, which is after all only
just, perfectly expresses Cuvier's attitude to the transcendental
theories, and was p