156 PLANT-ANIMALS [CH. 



its inception on nitrogen-hunger. It is tempting 

 to push this hypothesis to its limits, and to imagine 

 that the great saprophytic groups of the fungi 

 and the bacteria owe their origin to the changed 

 mode of nutrition imposed upon them by lack of 

 nitrogen. That the fungi are examples of descent 

 by reduction is undisputed. All the evidence points 

 to their derivation from chlorophyll-containing algal 

 ancestors. Having lost their chlorophyll, and, with 

 it, their powers of photosynthesis, they are now con- 

 demned to obtain both carbon and nitrogen in the 

 form of organic compounds and hence are compelled, 

 with the bacteria, to play the part of Nature's 

 scavengers. In their quest for food, they settle 

 either on the dead remains of plants or animals, 

 or, invading the living organism, they exchange a 

 saprophytic for a parasitic mode of life. 



The hypothesis suggested here is that the first 

 and fatal step from independence to dependence was 

 the outcome of the nitrogen scarcity which exists 

 in Nature. Confronted with inadequate supplies of 

 nitrogen, the photosynthetic activity of their chloro- 

 phyll apparatus was brought to a standstill. The 

 organisms, unable to obtain a sufficiency of inorganic 

 nitrogen compounds, were constrained to resume their 

 powers, never wholly lost, of absorbing nitrogen com- 

 pounds in organic form. But such organic nitrogen- 

 containing compounds contain also carbon. Hence 



