THE LIVING WORLD 189 



definite relations with past time as well as with space ; 

 and it is evident that as age has succeeded age, there 

 has been a process of replacement in vegetable and 

 animal forms, new kinds having come into being one 

 after another. It seems also evident that in the earliest 

 ages the world was entirely devoid of living creatures. 



Furthermore, animals and plants have also definite rela- 

 tions with each other. If a beast of prey finds its way 

 into a region peopled with creatures good for food, it 

 will increase and multiply to their detriment ; while the 

 most peaceful animal will suffer, by the introduction, 

 into a limited area, of creatures which are rivals because 

 they feed upon the same food, the supply of which will, 

 sooner or later, be insufficient for all. Here the reader 

 may ask, if the world was once without life, whence did 

 life come ? and what is life ? Also, since new kinds 

 have replaced older ones which disappeared, the question 

 naturally arises, how did new kinds arise ? 



But in a work like the present one (which is but an 

 introduction to the elements of science), the con- 

 sideration of such questions would be out of place. 

 They would be as much out of place a,s would be a 

 consideration of the questions "What is heat?" and 

 " What is light ? " As to the latter questions we have 

 provisionally noted certain useful working hypotheses. 

 Similarly, since up to the present day, life has not been 

 evolved by us from inorganic matter, we may, as a work- 

 ing hypothesis, adopt the belief that life is the energy 

 of a peculiar form of force which exists differently in 

 each different kind of organism, and that this force is a 

 main agent in the development of new kinds. As to 

 the first introduction of life on the surface of this planet 

 our reason is as yet entirely in the dark. 



Tho number of all the various kinds of living 



