6 ELEMENTS OF BIOLOGY, 



lies them that life is inseparably connected with organisa- 

 tion. More recently attempts have been made to prove 

 that life is merely a form of energy or motion, in which case 

 no difficulty should be found in giving it an exact defini- 

 tion. In the meanwhile, however, this view certainly can- 

 not be said to have been satisfactorily proved, and it does 

 not appear that any rigid definition of life is possible. We 

 may therefore employ the name life as a collective term 

 for the 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. 



As regards the conditions under which alone life or vital 

 activity can be manifested, we have to consider two sets of 

 conditions : the intrinsic or indispensable conditions, \vith- 

 out which no vital phenomena are possible; and the extrin- 

 sic conditions, which are generally present, but which do 

 not appear to be actually essential to living beings. Under 

 the first head, we have only to consider the presence of a 

 " physical basis ; " under the second head, we may briefly 

 look to the presence of organisation, light and air, and the 

 necessity for a certain temperature. 



a. Protoplasm. — The first of the questions as to the con- 

 ditions of life which it. is necessary to consider, is whether 

 the phenomena of vitality are necessarily associated with 

 any particular form of matter, or with any special " physical 

 basis," as it has been aptly termed. The answer to this 

 question may with little hesitation be given in the affirma- 

 tive. It does not at all appear that the phenomena of life 

 can be manifested by any and every fomi of matter ; and a 

 very little reflection ought to convince us that it would be 

 very surprising if the reverse of this were the case. There 

 is no physical or chemical force which can be rendered 

 manifest to us by all and sundry forms of matter, and it 

 would be indeed remarkable if the case were otherwise with 

 the forces of the living organism. When, for example, we 

 say that certain forms of matter, such as the metals, are 



