THE ANIMAL KINGDOM. 139 



The difficulty of framing a definition arises from the 

 fact that both animals and plants are members of the liv- 

 ing world, and hence have many features in common, 

 which may be summarized in the expression that they 

 are alive. We do not know what life * is ; we only know 

 it by the phenomena which it exhibits, which may be 

 briefly stated as follows: 



All living beings are composed of a peculiar substance 

 (or group of substances) called protoplasm, and this proto- 

 plasm is known only as the product of life. When un- 

 mixed with other substances it is semifluid, transparent, 

 and slightly heavier than water. It consists of a large 

 number of chemical elements carbon, oxygen, hydrogen, 

 nitrogen, sulphur, and phosphorus predominating but 

 how these are arranged is as yet one of the mysteries. 

 When treated with the reagents of the chemist it dies 

 and is no longer protoplasm. 



Protoplasm, and consequently the animals and plants 

 which contain it, exhibits certain properties. It can take 

 non-living substances and convert them into a part of itself, 

 that is, make them alive. The bread and the roast-beef 

 which we eat are dead; yet we know that thev become 



/ J ^ 



parts of ourselves, not in the shape gf bread and roast-beef, 

 but as our own protoplasm. This process is known as 

 assimilation, and continued assimilation results in growth. 

 A snowball grows by accretions on the outside, but the 

 growth of animals and plants occurs all through the body 

 and throughout every part of it. It is a growth of the 

 protoplasm. 



* Frequently the expression 'vital force' is used, as if there were 

 some distinct force in nature exhibiting itself only in living forms. 

 This is entirely unnecessary, for each and every phenomenon of life 

 can be explained by physical and chemical means. 



