194 ELEMENTS OF SCIENCE 



heat, as they may also do from imbibing moisture ; but 

 living protoplasm has an apparently spontaneous power 

 of contraction and expansion under certain external 

 conditions which do not occasion such movements in 

 inorganic matter. 



Under favouring conditions, protoplasm has a power 

 of performing chemical changes, which result in producing 

 heat far more gently and continuously than it is produced 

 by the combustion of inorganic bodies. Thus it is that 

 the heat is produced which makes its presence evident to 

 us in what we call " warm-blooded animals," the most 

 warm-blooded of all being birds. 



Protoplasm has also the wonderful power of trans- 

 forming certain adjacent substances into material like 

 itself into its own substance and so, in a sense, 

 creating a new material. Thus it is that organisms have 

 the power to nourish themselves and grow. An animal 

 would vainly swallow the most nourishing food if the 

 ultimate, protoplasmic particles of its body had not this 

 power of "transforming" suitable substances brought 

 near them in ways to be hereinafter noticed. 



Without that, no organism could ever " grow." The 

 growth of organisms is utterly different from the increase 

 in size of inorganic bodies. Crystals, as we have seen,* 

 grow merely by external increment ; but organisms grow 

 by an increment which takes place in the very innermost 

 substance of the tissues which compose their bodies and 

 the innermost substance of the cells which compose 

 such tissues; this peculiar form of growth is termed 

 intussusception. 



Protoplasm, after thus augmenting its mass, has a 

 further power of spontaneous division, whereby the mass 



* See ante, pp. 143 and 144. 



