MATTER AND CELLS 13 



direction and with regulated strength to accomplish an 

 end, as when a particle of material suitable for building 

 up a cell is drawn in and used for that purpose. 



8. Plants and Animals. No naturalist can at the pres- 

 ent day place his finger upon a line of separation and 

 say: All living things upon this side are plants; all upon 

 that side, animals. It is, indeed, easy to distinguish the 

 higher forms of animal life from the higher forms of plant 

 life, and the most striking difference is that the animal 

 possesses the power of spontaneous movement, while the 

 plant is rooted to one spot. Other distinctions appear as 

 the two forms of life are studied. For example, both are 

 dependent for their continued life and growth upon the 

 food which is supplied from without themselves ; but 

 plants (the fungi excepted) subsist mainly upon carbon 

 dioxide, water, and mineral salts, while animals live upon 

 water and those chemical compounds which have formed 

 part of living bodies, that is, organic materials. Ani- 

 mals cannot use mineral substances as food except as they 

 are mixed with organic matter. But the simplest forms 

 of plant and animal life cannot be distinguished with 

 positiveness from each other. Both consist of single pro- 

 toplasmic cells, and it is not possible to show that the proto- 

 plasm of one is essentially different from that of the other. 



As animals rise in the scale of being, however, they are 

 found to develop, as plants do not, a nervous system of 

 ever-increasing complexity and importance. Hence man, 

 as an animal, may be said to be distinguished from all 

 other animals by the superiority of his nervous system ; 

 and all the other parts of the human body may be consid- 

 ered as created simply to minister in some way to that 

 superior portion of the human frame which is the direct 

 agent or instrument of the highest manifestations of life. 



