PRELIMINARY DISCOURSE 15 



we speak of species among living bodies. Afterwards, when I have 

 treated of the general principles which concern animals, I shall adduce 

 proof of the degradation of organisation which runs through the entire 

 animal scale, placing the most perfect animals at the anterior extremity 

 of that scale. On the other hand I shall show the influence of en- 

 vironment and habit on the organs of animals, as being the factors 

 which favour or arrest their development. I shall conclude this 

 division by a discussion of the natural order of animals, and by an 

 account of their most suitable arrangement and classification. 



In the second division I shall put forward my ideas as to the order 

 and state of things which constitute the essence of animal Ufe ; and I 

 shall indicate the conditions necessary for the existence of this wonder- 

 ful natural phenomenon. Afterwards, I shall endeavour to ascertain 

 the exciting cause of organic movements ; of orgasm and of irrita- 

 bihty ; the properties of cellular tissue ; the sole condition under 

 which spontaneous generation can occur ; the obvious effects of vital 

 actions, etc. 



Lastlv. the third division will state my opinion as to the physical 

 causes of feehng, of the power to act, and of the acts of the intelligence 

 found in certain animals. 



In this di\àsion I shall treat : 1st, the origin and formation of the 

 nervous system ; 2nd, the nervous fluid, which can only be known 

 indirectlv, but whose existence is attested by phenomena that it 

 alone can produce ; 3rd, physical sensibility and the mechanism 

 of sensations ; 4th, the reproductive power of animals ; 5th, the 

 origin of the will and the faculty of wilUng ; 6th, ideas and the different 

 kinds of them ; 7th, lastly, certain peculiar acts of the understanding, 

 such as attention, thoughts, imagination, memory, etc. 



The reflections set forth in the 2nd and 3rd divisions doubtless 

 comprise subjects that are very difficult to examine, and may 

 even appear insoluble ; but they are so full of interest that such 

 attempts may possibly be profitable, either in the disclosure of un- 

 perceived truths or in pointing out the direction in which they may 

 be sought. 



