EFFECT OF THE APPEARANCE OF CHLOROPHYLL 397 



like the animals. Their nutrition and metabolism are 

 intermediate between the two, since they can use 

 simpler nitrogenous foods than animals can, and though 

 they grow much more quickly than green plants they 

 tend, like the latter, to form an excess of carbohydrates. 

 Some authorities believe that the fungi are all derived 

 in evolution from green plants through the loss of 

 chlorophyll, but we have no sufficient grounds for 

 certainty. The fungi may have been derived, like the 

 bacteria, directly from primitive forms which had a 

 type of nutrition intermediate between that of animals 

 and that of green plants. 



We have thus seen that the starting points of the 

 great primary differentiation of living beings into 

 animals and plants must have depended upon differences 

 in the constitution of the protoplasm of the earliest 

 forms of life, just as the differences between species in 

 existing organisms must ultimately depend upon differ- 

 ences between their protoplasms. Some organisms 

 happened to produce the complex pigment chlorophyll 

 (p. 112), which by the absorption of light enabled the 

 protoplasm to use carbon dioxide as food to assimilate 

 carbon in that simple form. Such organisms differed 

 from those which did not produce chlorophyll perhaps 

 in no other respect, 1 and yet so small a difference has 

 eventually led, in the course of long ages, to the differ- 

 ence between a man and an oak tree. And this is the 

 story of all evolutionary development. The widest 

 differentiations have their origin in such small differ- 

 ences, of the order of specific differences, which are 

 just sufficient to give the necessary bias to wholly 



1 We know existing unicellular organisms which differ from one 

 another in just this way, and consequently, while almost iden- 

 tical in form and structure, have different habitats and modes of 

 feeding. 



