INTRODUCTION. 11 



general laws. He is not, like the experimenter, allowed to subtract 

 them successively from each condition, and to reduce the problem 

 to its elements he is compelled to take it in its entireness, with all 

 its conditions at once, and can perform the analysis only in thought. 

 Suppose, for example, we attempt to insulate the numerous pheno- 

 mena which compose the life of any of the higher orders of animals; 

 a single one being suppressed, every vestige of life is annihilated. 



Dynamics have thus nearly become a science of pure calculation, 

 Chemistry is still a science of pure experiment; and Natural History, 

 in a great number of its branches, will long remain one of pure 

 observation. 



These three terms sufficiently designate the methods employed in 

 the three branches of the natural sciences; but in establishing be- 

 tween them very different degrees of certitude, they indicate, at the 

 same time, the point to which they should incessantly tend, in order 

 to attain nearer and nearer to perfection. 



Calculation, if we may so express it, thus commands Nature, and 

 determines her phenomena more exactly than observation can make 

 them known; experiment compels her to unveil; while observation 

 pries into her secrets when refractory, and endeavours to surprise 

 her. 



There is, however, a principle peculiar to Natural History, which 

 it uses with advantage on many occasions; it is that of the conditions 

 of existence, commonly styled final causes. As nothing can exist 

 without the re-union of those conditions which render its existence 

 possible, the component parts of each being must be so arranged as 

 to render possible the whole being, not only with regard to itself 

 but to its surrounding relations. The analysis of these conditions 

 frequently conducts us to general laws, as certain as those that are 

 derived from calculation or experiment. 



It is only when all the laws of general physics and those which 

 result from the conditions of existence are exhausted, that we are 

 reduced to the simple laws of observation. 



The most effectual method of obtaining these is that of compa- 

 rison. This consists in successively observing the same bodies in the 

 different positions in which nature places them, or in a mutual com- 

 parison of different bodies; until we have ascertained invariable 

 relations between their structures and the phenomena they exhibit. 

 These various bodies are kinds of experiments ready prepared by 

 Nature, who adds to or deducts from each of them different parts, 



