4 MANUAL OF ZOOLOGY. 



seat of something in virtue of which the living body can over- 

 ride the physical laws which control all dead matter. The 

 living body is the seat of energy, and can overcome the pri- 

 mary law of the inertia of matter. It has certain relations 

 with the €uter world other than those of mere passivity. 

 However humble it may be, and even if it be permanently 

 rooted to one place, some part or other of every living body 

 possesses the power of spontaneous and independent move- 

 ment — a power possessed by nothing that is dead. 



3. Nature of Life. 



We have next to determine — and the question is one of 

 great difficulty — ^what connection exists between organisation 

 and life. Is organisation, as we have defined it, essential to 

 the manifestation of life, or can vital phenomena be exhibited 

 by any body which is devoid of an organised structure ? In 

 other words, is life the cause of organisation, or the residt of it ? 

 And first, what do we mean by 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 phenomena with 

 diversity of external influences ; " Duges, as " the special acti- 

 vity of organised bodies ; " and Beclard, as " organisation in 

 action." All these definitions, however, are more or less objec- 

 tionable, since the assumption underlies them all that life is 

 inseparably connected 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. 



As regards the connection between life and organisation, 

 it appears that whilst all organised bodies exhibit this ten- 

 dency to change, and are therefore alive, all living beings are 

 not necessarily organised. Many of the lowest forms of life 

 (such as the Foraminifera amongst the Protozoa) fail to fulfil 

 one of the most essential conditions of organisation, being de- 

 void of definite parts or organs of any kind. Nevertheless, they 

 are capable of manifesting all the essential phenomena of life ; 

 •they are produced from bodies like themselves; they eat, 

 digest,_ and move, and exhibit distinct sensibility to many ex- 

 ternal impressions. Furthermore, many of these little masses 

 of structureless jelly possess the power of manufacturing for 

 themselves, of lime, or of the still more intractable flint, 



