INTRODUCTION. 17 



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. 



DiflTerences of this kind, between organized beings, form what 

 are termed varieties. 



There is no proof, that all (he differences which now distinguish 

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

 stances. All that has been advanced upon this subject is hypothe- 

 tical. 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 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 acciden- 

 tal subdivisions of species. 



Species should be defined, the re-union of individuals descended 

 one from the other, or from common parents, or from such as resem- 

 ble them, as strongly as they resemble each other. But although this 

 definition is strict, it will be seen that its application to particular 

 individuals may be very difficult, where the necessary experiments 

 have not been made. 



Thus then it stands — absorption, assimilation, exhalation, develop- 

 ment and generation are functions common to all living bodies; birth 

 and death the universal limits of their existence; an areolar, con- 

 tractile tissue, containing within its laminae fluids or gases in motion, 

 the general essence of its structure; substances almost all suscepti- 

 ble of conversion into fluids or gases, and combinations capable of 

 an easy and mutual transformation, the basis of their chemical com- 

 position. Fixed forms that are perpetuated by generation distinguish 

 their species, determine the complication of the secondary 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 neither 

 produced nor changed by their own agency — life supposes their ex- 

 istence, its flame can only be kindled in an organization already 

 prepared, and the most profound meditation and lynx-eyed and deli- 

 C 



