BASIS OF THE PKOBLEM 59 



can serve as food for animals and survive. Nevertheless the 

 young, especially in the form of seeds, are very subject to attack 

 by animals ; seeds are one of the principal forms of food for 

 many animal species, and this form of elimination plays a large 

 part among plants. Dependence upon suitable organic and 

 inorganic surroundings plays much the same role as it does among 

 animals. Some species can only flourish in the shade of trees and 

 others in the open. Some species require one kind of soil and 

 others another. The importance of external circumstances again 

 is very similar. 



There is another factor of a somewhat different nature which 

 is of great importance. ' With plants ', says Darwin, ' there is 

 a vast destruction of seeds, but, from some observations which 

 I have made, it appears that the seedlings suffer most from 

 germinating in ground already thickly stocked with other plants.' 1 

 In order that a seed may germinate, it must not only fall upon 

 suitable soil but must find enough suitable soil unoccupied. 

 Otherwise it will not germinate or will not develop into an adult. 

 This is different from anything which happens among animals, 

 and is clearly an approach to starvation. li is, however, better 

 thought of as the result of the inability of any given area to 

 support more than a given amount of life. The endowment of 

 any area may be such as to render it incapable of supporting life 

 at all, or it may be such as to render it capable of supporting 

 any degree of life up to and beyond that for which there is space. 

 In the sea the amount of nitrogen is the limiting factor, and the 

 deficiency of nitrogen is such that the question of space does not 

 arise. 2 In many parts of the world's surface, however, the endow- 

 ment is such that more plants could be supported than there is 

 space for. 



In this connexion it may be noted that the limitation of the 

 surface of the earth is not a cause of elimination in the true sense. 

 This is best seen if we imagine the surface to be extended. If 

 to a continent already inhabited there is added an unoccupied 

 area, there will, if the new area is generally of the same nature 

 as that alone formerly existing, be a spreading of organic life 

 over the new area. It will only be at the fringe of the occupied 

 area that there will be any difference in the amount of elimina- 



1 Darwin, Origin of Species, p. 49. 2 Johnstone, Conditions of Life in the 



Sea, p. 235. 



