IN NATURAL HISTORY. 11 
It is not upon any special features, tnen, that 
these largest divisions of the animal kingdom are 
based, but simply upon the general structural 
idea. Striking as this statement was, it was cold- 
ly received at first by contemporary naturalists: 
they could hardly grasp Cuvier’s wide generaliza- 
tions, 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 key-note to the natural affinities 
among animals. Lamarck, his contemporary, 
not recognizing the truth of this principle, dis- 
tributed the animal kingdom into two great di- 
visions, which he calls Vertebrates and Inverte- 
brates. Ehrenberg also, at a later period, an- 
nounced another division under two heads, — 
those with a continuous solid nervous centre, 
and those with merely scattered nervous swell- 
ings.* 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 
Anauna. 
This coincidence between systems based on 
* For more details upon the systems of Zoodlogy, see Agassiz’s 
Essay on Classification in his “Contributions to the Natural 
History of the United States,” Vol. I.; also printed separately. 
