8 INTRODUCTION. 
ing with the species, particular parts of which they may have been de- 
prived—this is called the power of reproduction. 
The developement of organized beings is more or less rapid, and more 
or less extended, as circumstances are more or less favourable. Heat, the 
abundance and species of nutriment, with other causes, exercise great in- 
fluence, and this influence may extend to the whole body in general, or to 
certain organs in particular: thence arises the impossibility of a perfect 
similitude between the offspring and parent. 
Differences of this kind, between organized beings, form what are termed 
varieties. 
There is no proof, that all the differences, which now distinguish er- 
ganized beings, are such as may have been produced by circumstances. 
All that has been advanced upon this subject is hypothetical. Experi- 
ence, on the contrary, appears to prove, that, in the actual state of the 
globe, varieties are confined within rather narrow limits, and go back as 
far as we may, we siill find those limits the same. 
We are thus compelled to admit of certain forms, which, frem the origin 
of things, have perpetuated themselves without exceeding these limits, and 
every being appertaining to one or other of these forms constitutes what 
is termed a species. Varieties are accidental subdivisions of species. 
Generation being the only means of ascertaining the limits to which va- 
rieties may extend, species should be defined—the re-union of individuals 
descended one from the other, or from common parents, or from such as 
resemble them as strongly as they resemble each other. But although 
this definition is strict, it will be seen that its application to particular in- 
dividuals may be very difficult, where the necessary experiments have not 
been made. 
Thus then it stands—absorption, assimilation, exhalation, developement, 
and generation, are functions common to all living bodies; birth and death 
the universal limits of their existence; an areolar, contractile tissue, con- 
taining within its lamine fluids or gases in motion, the general essence of 
its structure; substances, almost all susceptible of conversion into fluids or 
gases, and combinations capable of an easy and mutual transformation, the 
basis of their chemical composition. Fixed forms that are perpetuated by 
generation distinguish their species, determine the complication of the se- 
condary functions proper to each of them, and assign to them the parts 
they are to play on the great stage of the universe. These forms are nei- 
ther produced nor changed by their own agency—life supposes their exist- 
ence, its flame can only be kindled in an organization already prepared, 
and the most profound meditation and lynx-eyed and delicate observation 
can penetrate no farther than the mystery of the pre-existence of germs. 
