INTRODUCTION. 



598. The cerebral forces and the vis nervosa are essential 

 constituents of the proper animal nature of animal organisms 

 (6, 356)^ and in the more perfect animals are in close connection 

 (591 — 593). The aggregate of those animal forces, which act na- 

 turally in connection in an animal body, constitute its whole 

 animal nature, and this whole is now to be considered; its two 

 essential constituents even in the most perfect, having been dis- 

 cussed in the two preceding parts. 



599. The arrangement of this part is as follows : — first, the 

 character of the whole animal nature of an animal will be 

 sketched ; secondly, the existence of distinct classes of animals 

 will be proved ; then the origin of animals according to their 

 nature will be considered ; next, animal life and its natural 

 periods up to its full development ; then the system of forces 

 necessary to animal life, or how they act with and through each 

 other for its maintenance and perfection ; and, finally, old age 

 and death will be treated of in succession. 



