THE ANIMAL KINGDOM. 339 



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

 ing world, and hence have many features in common, which 

 may be summarized in the expression that both 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) known as 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 contains 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 proto- 

 plasm. 



This 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 they become 

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

 but as our own protoplasm. This process is known as 

 assimilation, and continued assimilation results in growth. 

 A snow-ball 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. 



Protoplasm has the power of spontaneous motion. Under 



* 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, 



