NATURE OP LIFE. 3 



are not purely physical or chemical, and they show no ten- 

 dency to periodical vicissitudes. Organised bodies are pre- 

 eminently distinguished by the tendency which they show to 

 pass through spontaneous and cyclical changes. 



To sum up, all bodies which are composed of an aggrega- 

 tion of diverse but definitely related parts, which have a defi- 

 nite shape, bounded by curved lines and presenting concave 

 and convex surfaces, which increase in size by the intussus- 

 ception of foreign particles, and which pass through certain 

 cyclical changes, are organised ; and it is with the study of 

 bodies such as these that Biology is concerned. 



3. NATURE OP 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 denned 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 result 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, 



u2 



