30 BIOLOGY. [BOOK i. 



the principal vital phenomenon, that which serves as a support 

 to all the others, is a double movement of assimilation and of dis- 

 assimilation, of renovation and of destruction, in the midst of living 

 matter ; that this matter may be either in a semi-solid state, and 

 without structure, as in certain inferior organisms; or that it 

 may be in a liquid state more or less viscous, like the blood and 

 lymph of the superior animals ; or finally, that it may be 

 modelled into anatomical elements, into cells and fibres bathed 

 with liquids and gases, as in the bodies of all the superior animal 

 and vegetal organisms. 



The living substance is thus a chemical laboratory in constant 

 action. It is the physical or chemical properties of this sub- 

 stance, diversely modified, which underlie all the vital properties, 

 nutrition, growth, reproduction, the chlorophyllian attribute, motility, 

 and innervation. \ 



Now the six properties w^ich we have just enumerated are 

 the six principal modes of living activity, the six categories 

 under which all biological phenomena group and class them- 

 selves. The chlorophyllian property is almost exclusively vegetal ; 

 but the five other fundamental properties represent, when united, 

 the highest, the most complete expression of life. But they are 

 far from being always united ; they are also far from having the 

 same importance. Some of them are primordial, some secondary. 



The most important of all is evidently nutrition, the double 

 and perpetual movement of molecular renovation of the living 

 substance. Without nutrition there can be no growth, no repro- 

 duction, no movement, no conscious sensitiveness, no thought. 



In truth, life can be conceived of as reduced to its most simple 

 expression, to mere nutrition. A being capable of nourishing 

 itself, and destitute of every other property or function, lives 

 still ; but if it has not the faculty of reproduction, which, as we 

 shall see, is only a simple extension of the nutritive property, 

 its life will be only an individual life ; a moment will come 

 when the nutritive exchanges will slacken, when the nutritive' 

 residue, incompletely expulsed, will impregnate the living tissues 



