IN NATURAL HISTORY. 11 



It is not upon any special features, then, that 

 these largest division's 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 

 Anaima. 



This coincidence between systems based on 



* For more details upon the systems of Zoology, see Agassiz's 

 Essay on Classification in his "Contributions to the Natural 

 History of the United States," Vol. I. ; also printed separately. 



