24 THE LIVING WORLD. 



from its dead condition into its own substance. Now 



^x* 



it is not possible to imitate many of these changes as 

 yet in our laboratories, but that they are all chemical 

 changes can scarcely be questioned. Even the con- 

 structive changes by which the body raises the 

 compounds into a plane of greater complexity, must 

 be regarded simply as chemical processes, the energy 

 which is required for them being obtained by the 

 breaking down of other portions of food. And in 

 plants also growth must be regarded as a chemical 

 process, for it consists in the combination of simple 

 organic compounds to form complex ones under the 

 influence of sunlight. 



The third property of living matter reproduction 

 seems at first sight to be a more marvellous power 

 than that of growth ; but most biologists think that 

 it is easily derived from the latter. Fundamentally, 

 reproduction is a direct and necessary result of 

 growth. In its simplest form, as found in the uni- 

 cellular animals, it is seen to be nothing more than a 

 division. The unicellular organism by chemical pro- 

 cesses continues to assimilate food and thus to grow. 

 It keeps on increasing in size until it finally becomes 

 so large that the cohesion between its parts is in- 

 sufficient to keep the great bulk together, and as a 

 result it divides into two parts. Each of these parts 

 is of course like the other, and there are thus two 

 organisms where there was only one before. This is 

 the simplest case of reproduction, and heredity in 

 this case is to be explained as a necessary result of 

 growth. Now, it is not difficult to see how the more 

 complex forms of reproduction may have been de- 



