126 LIFE ON THE EARTH. 



any use. 1 They omit, moreover, that which is the very 

 essence of life, viz., that of reproducing life more or 

 less like in kind to itself. No definition can be good 

 which does not include the condition of an organisation 

 capable, by sexual or other means, of such repro- 

 duction. 



The definition of life has been perplexed by other 

 ideas annexed to it. The terms ' vital principle,' ' vital 

 energy/ ' vital force,' &c., though needed for descrip- 

 tion, and which in one sense may be admitted as 

 realities, do not really define anything that we can 

 construe to the understanding. We cannot assert, on 

 proof, that life is engendered by, or engenders, any 

 power or force special to itself. Nevertheless, in as- 

 suming, which we must do, that it transforms certain 

 known forces so as to appropriate them to its peculiar 

 functions, we virtually admit a special and character- 

 istic power, call it what we will. A formal definition 

 (which after all has very little scientific value) may 

 exclude what is thus ambiguous to our reason, but in 

 the present state of our knowledge the conception of 

 life embraces it by what is next to a necessity. 



Something more may be said for bringing time, as 

 an element, into the definition sought for. Every form 

 of life, endlessly dissimilar though these forms be, has 

 its average period and term of existence, as well as 

 chronometry in its various particular functions. Growth, 



1 The definition by an eminent philosopher of our own day that life 

 is ' the continuous adjustment of internal relations to external relations/ 

 is subject to the same charge of a generality which lessens its scientific 

 value. 



