INTRODUCTION. 3 



phenomena necessary to existence in any of the higher orders of 

 animals ; a single one being suppressed, every vestige of life is anni- 

 hilated. 



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 obser- 

 vation. 



These three terms sufficiently designate the methods employed in 

 studying the three branches of the natural sciences ; but in esta- 

 blishing between them very different degrees of certainty, they indicate, 

 at the same time, the point to which the two latter 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, and, when refractory, 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 Jinal causes. As nothing can exist unless it 

 combine within itself certain conditions which render its existence pos- 

 sible, 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 ; and 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 an analysis of them is that 

 of comparison. This consists in successively observing the same bodies 

 in the different positions in which nature places them, or in a mutual 

 comparison 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, just 

 as we might wish to do in our laboratories ; showing us, herself, at the 

 same time, their various results. 



In this way we finally succeed in establishing certain laws by which 

 these relations are governed, and which are employed like those that 

 are determined by the general sciences. 



The incorporation of these laws of observation with the general laws, 



B 2 



