IN NATURAL HISTORY. 9 
fondées sur le développement ou l’addition de 
quelques parties, qui ne changent rien & l’essence 
du plan.’’* 
The value of this principle was soon tested by 
its application to facts already known, and it was 
found that animals whose affinities had been 
questionable before were now at once referred 
to their true relations with other animals by as- 
certaining whether they were built on one or 
another of these plans. Of such plans or struc- 
tural conceptions Cuvier found in the whole ani- 
mal kingdom only four, which he called Verte- 
brates, Mollusks, Articulates, and Radiates. 
With this new principle as the basis of investi- 
gation, it was no longer enough for the naturalist 
to know a certain amount of features character- 
istic of a certain number of animals, —he must 
penetrate deep enough into their organization to 
* If we consider the animal kingdom according to the princi- 
ples advanced above, — freeing ourselves at the same time from 
prejudices founded on previously established divisions, and look- 
ing at animals only with reference to their nature and or- 
ganization, excluding their size, their utility, our greater or less 
familiarity with them, and all other accessory circumstances, — 
we shall find that there exist four principal forms, four general 
plans, if we may so express it, in accordance with which all 
animals seem to have been modelled, and the ulterior divisions 
of which, by whatever title naturalists may have dignified them, 
are only comparatively light modifications, founded on the de- 
velopment or the addition of some parts not affecting the essen- 
tial elements of the plan. 
1* 
