NATURE AND CONDITIONS OF LIFE. 7 



d. Reproduction. Every living body has the power of re- 

 producing its like. Directly or indirectly, every living body 

 has the power of giving off minute portions of its own sub- 

 stance, which, under proper conditions, will be developed into 

 the likeness of the parent. 



4. NATURE AND CONDITIONS OF LIFE. 



Life has been variously defined by different writers. Bichat 

 defines it as "the sum total of the functions which resist 

 death;" Treviranus, as "the constant uniformity of pheno- 

 mena with diversity of external influences ; " Duges, as " the 

 special activity of organised bodies ; " and Beclard, as " organ- 

 isation in action." All these definitions, however, are more or 

 less objectionable, since they either really mean nothing, or 

 the assumption underlies them that life is inseparably con- 

 nected with organisation. In point of fact, no rigid definition 

 of life appears to be at present possible, and it is best to regard 

 it as being simply a tendency exhibited by certain forms of 

 matter, under certain conditions, to pass through a series of 

 changes in a more or less definite and determinate sequence. 

 The essential phenomenon of vitality is, therefore, in the 

 words of Herbert Spencer, "the continuous adjustment of 

 internal relations to external relations," and life, in its effect ', 

 is the totality of the functions of a living being. Life, how- 

 ever, may also be considered as a cause, since amongst the 

 phenomena presented by all living beings there are some 

 which cannot be referred to the action of known physical or 

 chemical laws, and which, therefore, temporarily at any rate, 

 we must term " vital." 



Whilst the nature of life thus does not admit of rigid defini- 

 tion, we find that the phenomena of vitality can only be mani- 

 fested under certain conditions, some of these being intrinsic 

 and indispensable, whilst others are extrinsic, and not in them- 

 selves, or collectively, essential. 



The only intrinsic condition of life appears to be the exist- 

 ence of a special "physical basis" as it has been termed. We 

 do not find, namely, that the phenomena of vitality can be 

 manifested by any and every form of matter. On the contrary, 

 and as might have been expected upon a priori grounds, all 

 living bodies appear to be composed of a special substance, 

 which is the material basis of life, and which seems to be sub- 

 stantially identical in all alike. No living body is throughout 



