8 INTRODUCTION. 



ing with the species, particular parts of wliich tliey may liave been de- 

 prived — this is called the foiver 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 ofi'spring 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 or- 

 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 still find those limits the same. 



We are thus compelled to admit of certain forms, which, from 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 deatli 

 the universal limits of their existence ; an areolar, contractile tissue, con- 

 taining within its lamina 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. Fijied 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. 



