2 ZOOLOGICAL PHILOSOPHY 



creates the different special organs, and thus builds up the animal 

 organisation, special functions arise to a corresponding degree, and 

 in the most perfect animals these are numerous and highly developed. 



These reflections, which I was bound to take into consideration, 

 led me further to enquire as to what life really consists of, and what 

 are the conditions necessary for the production of this natural pheno- 

 menon and its power of dwelling in a body. I made the less resistance 

 to the temptation to enter upon this research, in that I was then con- 

 vinced that it was only in the simplest of all organisations that the 

 solution of this apparently difficult problem was to be found. For 

 it is only the simplest organisation that presents all the conditions 

 necessary to the existence of life and nothing else beyond, which might 

 mislead the enquirer. 



The conditions necessary to the existence of life are all present 

 in the lowest organisations, and they are here also reduced to their 

 simplest expression. It became therefore of importance to know 

 how this organisation, by some sort of change, had succeeded in giving 

 rise to others less simple, and indeed to the gradually increasing com- 

 plexity observed throughout the animal scale. By means of the two 

 following principles, to which observation had led me, I beUeved I 

 perceived the solution of the problem at issue. 



Firstly, a number of known facts proves that the continued use of 

 any organ leads to its development, strengthens it and even enlarges 

 it, while permanent disuse of any organ is injurious to its develop- 

 ment, causes it to deteriorate and ultimately disappear if the disuse 

 continues for a long period through successive generations. Hence 

 we may infer that when some change in the environment leads to a 

 change of habit in some race of animals, the organs that are less used 

 die away little by little, while those which are more used develop 

 better, and acquire a vigour and size proportional to their use. 



Secondly, when reflecting upon the power of the movement of the 

 fluids in the very supple parts which contain them, I soon became 

 convinced that, according as this movement is accelerated, the fluids 

 modify the cellular tissue in which they move, open passages in them, 

 form various canals, and finally create different organs, according 

 to the state of the organisation in which they are placed. 



Arguing from these two principles, I looked upon it as certain 

 that, firstly, the movement of the fluids within animals — a move- 

 ment which is progressively accelerated with the increasing complexity 

 of the organisation — and. secondly, the influence of the environment, 

 in so far as animals are exposed to it in spreading throughout all 

 habitable places, were the two general causes which have brought 

 the various animals to the state in which we now see them. 



