PLANTS AND ANIMALS 35 



The explanation of the difficulty really lies in the fact that 

 both plants and animals originally sprang from common unicel- 

 lular ancestors which were neither the one thing nor the other. 

 The first appearance of chlorophyll initiated the great cleavage 

 between the animal and vegetable kingdoms. Thenceforward 

 the two great groups developed each along lines of its own. By 

 virtue of their chlorophyll the green plants became the great 

 proteid manufacturers of the world, and the animals became 

 dependent upon them for their food supply. Animals are largely 

 dependent upon green plants in another respect also, for the 

 latter, as we have seen, split up the carbon dioxide, formed as a 

 waste product in the respiration of both groups, and thus set free 

 fresh supplies of the necessary oxygen. 



Closely correlated with the differences in their mode of nutri- 

 tion are the great differences in the mode of life of the higher 

 plants and animals. Plants have no need to move from place to 

 place in search of food, which they obtain from the air and the 

 soil, and the supplies of which are constantly renewed by wind 

 and rain. Highly organized animals, on the other hand, would 

 soon exhaust their supplies if they remained always in the same 

 place, and it is doubtless the necessity for actively seeking out 

 and even contending one with another for fresh supplies that 

 has brought about the wonderful elaboration and perfection of 

 their organization, while the fact that in so many cases they 

 devour one another, instead of remaining directly dependent upon 

 vegetable organisms, must have greatly intensified the struggle 

 for existence and correspondingly increased the rate of progress 

 in their evolution. 



D 2 



