8 INTRODUCTION. 



any external cause. In fact, life seems to exercise upon the elements 

 which constitute the living body, an action contrary to that which the 

 usual chemical affinities would produce, were life extinct. It seems, 

 therefore, impossible for these affinities to produce the living principle ; 

 and yet we know of no other power in nature capable of reuniting 

 previously separated particles. 



The birth of organised beings is, therefore, the greatest mystery of 

 the organic economy and of all nature : we see them developed, but 

 we never see them being formed ; nay, more, all those whose origin we 

 can trace, have at first been attached to a body similar in form to their 

 own, but which was developed before them ; in a word, to a parent. 

 So long as the offspring has no independent existence, but participates 

 in that of its parent, it is called a germ. 



The place to which the germ is attached, and the cause which 

 detaches it and gives it an independent life, vary ; but this primitive 

 adhesion to a similar being is a rule without exception. The separa- 

 tion of the germ is called generation. 



Every organised being reproduces others that are similar to itself; 

 otherwise, death being a necessary consequence of life, the species would 

 soon become extinct. Organised beings have even the faculty of 

 reproducing, in degrees varying with the species, particular parts of 

 which they may have been deprived this is called the power of repro- 

 duction. 



The development of organised 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 

 influence, 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 organised beings, form what are termed varieties. 



There is no proof, that all the differences which now distinguish 

 organised beings are such as may have have been produced by circum- 

 stances. All that has been advanced upon this subject is hypothetical. 

 Experience, 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 these 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 subdivi- 

 sions of species. 



Species should be defined I he assemblage of individuals descended one 



