OF ANIMALS. 47 



however, are produced by irritation, but irritability in these 

 animals is dependent upon the nervous system. We also ob- 

 serve in them, and especially in insects, a faculty called in- 

 stinct^ and which, like an irresistible impulse, causes them to 

 produce, without being taught and without imitation, very 

 complicated actions, that are necessary to their preservation 

 and to that of their species. The vertebrated animals besides 

 irritability, sensibility, voluntary movements and instinct, 

 have cerebral functions which, to a certain degree, resembles 

 intellect. 



31. The varieties or the degrees of complications which 

 exist in each apparatus or function, are combined in various 

 modes, which constitutes the varieties of the general orga- 

 nization. The combination or the coexistence of the vari- 

 ous apparatuses of organs is determined; a certain state of 

 the nutritive or genital organs requiring, for the support of 

 life, some corresponding state of the organs of motions, of 

 sensibility, &c. According to a well defined distinction of or- 

 ganization, animals are divided into vertebrate and inverte- 

 brate. Man belongs to the former of these divisions. 



32. Although the invertebrata differ greatly from man, 

 their study is nevertheless of great utility to the anatomist and 

 physiologist ; we observe in them organization and life in 

 their greatest simplicity? and under a multitude of varieties. 

 They differ so much with each other, that they have no com- 

 mon and positive character. According to their organization, 

 they are divided into three great sections which differ from 

 each other as much as they are unlike the vertebra: these are 

 the radiated animals, the mollusca, and the articulata; and we 

 find even besides these three divisions, a class of very ques- 

 tionable beings that zoologists describe under the name of in- 

 fusorii, and which botanists claim as belonging to the con- 

 ferva. 



33. These dubious and microscopical animals, have a very 

 simple organization, different forms, and sometimes changea- 

 ble; they are homogeneous, transparentand diffluent; they have 

 no cavity, no distinct organ; they move, however, in the water 

 8 



