2 ELEMENTS OF BIOLOGY. 



of the leading principles upon which the sciences of Zoology 

 and Botany are based. With this view, technicalities will 

 be as far as may be avoided; and on all matters which are 

 still undecided the evidence on both sides will as far as 

 possible be given, so that the reader may be enabled to 

 form his own judgment as to the questions at issue. 



DIFFERENCES BETWEEN DEAD AND LIVING BODIES. 



In marking out the boundaries which limit the province 

 of Biology, the first point is obviously to arrive at a clear 

 conception as to the differences which separate all living 

 bodies from those that are dead. The leading characters 

 by which living bodies are distinguished from dead bodies 

 may be summed up as follows : Firstly, Every living body 

 possesses the power of taking into its interior certain ma- 

 terials foreign to those composing its own substance, and 

 of converting these into the materials of which its body 

 is built up. This constitutes the process of "assimilation," 

 and it is in virtue of this that living bodies grow. In 

 all cases alike, the materials to be assimilated are taken 

 into the interior of the body, and the process of growth is, 

 therefore, one depending upon the "intussusception" of 

 foreign matter in contradistinction to its mere addition 

 from the outside. 



When, on the other hand, dead bodies increase in .size, 

 as crystals do, the increase is produced simply by the addi- 

 tion of fresh particles from the exterior, or, as it is techni- 

 cally called, by the ''accretion" of matter. This process 

 cannot properly be considered as one of " growth," as 

 being wholly destitute of the essential element of a pre- 

 vious " assimilation." 



Secondly, All the actions of living beings are accompanied 

 by a corresponding destruction of the matter by which 

 these actions are manifested. In other words, partial 

 death is a constant accompaniment of life ; and the inces- 

 sant loss of substance eaused by vital action has to be 



