It is not upon any special features, then, that these largest divisions of
the animal kingdom are based, but simply upon the general structural idea.
Striking as this statement was, it was coldly received at first by
contemporary naturalists: they could hardly grasp Cuvier's wide
generalizations, and perhaps there was also some jealousy of the grandeur
of his views. Whatever the cause, his principle of classification was not
fully appreciated; but it opened a new road for study, and gave us the
keynote to the natural affinities among animals. Lamarck, his
contemporary, not recognizing the truth of this principle, distributed the
animal kingdom into two great divisions, which he calls _Vertebrates_ and
_Invertebrates_. Ehrenberg also, at a later period, announced another
division under two heads,--those with a continuous solid nervous centre,
and those with merely scattered nervous swellings.[3]

[3] For more details upon the different systems of Zoölogy, see
Agassiz's Essay on Classification in his _Contributions to the
Natural History of the United States_, Vol. I.

But there was no real progress in either of these latter classifications,
so far as the primary divisions are concerned; for they correspond to the
old division of Aristotle, under the head of animals with or without
blood, the _Enaima_ and _Anaima_. This coincidence between systems based
on different foundations may teach us that every structural combination
includes certain inherent necessities which will bring animals together on
whatever set of features we try to classify them; so that the division of
Aristotle, founded on the circulating fluids, or that of Lamarck, on the
absence or presence of a backbone, or that of Ehrenberg, on the
differences of the nervous system, cover the same ground. Lamarck
attempted also to use the faculties of animals as a groundwork for
division among them. But our knowledge of the psychology of animals is