INTRODUCTION. oO 
mistry 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 between 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 deter- 
mines 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 cer- 
tain 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 comparison. 
This consists in successively observing the same bodies in the different 
positions in which nature places them, or in a mutual comparison of dif- 
ferent 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 labo- 
ratories; shewing 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, 
either directly or by the principle of the conditions of existence, would 
complete the system of the natural sciences, in rendering sensible in all 
its parts the mutual influence of every being. To this end, should these 
who cultivate these sciences direct all their efforts. 
All researches of this nature, however, pre-suppose means of distin- 
guishing clearly, and causing others to distinguish, the bodies they are 
occupied with; otherwise we should be continually confounding them. 
Natural history then should be based on what is called a system of nature— 
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