SECTION II 

 COMMUNAL LIFE OF ORGANISMS 



CHAPTER XXII. RECIPROCAL RELATIONS AMONG 



ORGANISMS 



The non-living (physical, chemical) and other factors dealt with in 

 Section I do not suffice to impart a full comprehension of the production 

 of communities in the vegetable kingdom. 



On p. 84 mention is made of another factor the competition among 

 species of plants the importance of which is so great that many species 

 are excluded from great areas on the Earth, not by direct interference 

 on the part of non-living factors, but by the indirect interference involved 

 in competition for food with other stronger species. 



Another factor, animal-life, also has a powerful influence upon the 

 kind and the economy of vegetation. We have discussed the parts 

 played by earthworms, insects, and other small animals in causing physical 

 and chemical changes in soil, but animal-life affects the existence of 

 plants in many other ways, and among all living beings man stands in 

 the foreground as inducing the greatest modifications in plant-communi- 

 ties and in their reciprocal struggles. 



The manifold, complex, mutual relations subsisting among organisms 

 are matters of such profound import to plant -life and plant-communities 

 that this Section of our book is set apart for their consideration. 



CHAPTER XXIII. INTERFERENCE BY MAN 



Very diversified are the reciprocal relations between the plant -world 

 and mankind. Although the plant-world affects the human race, it is 

 itself to a far greater extent influenced by mankind ; indeed, vegetation 

 is the result of man's influence to such an extent that soon there will 

 be but few places upon earth where he has not modified or destroyed 

 the vegetation by directly turning it to his own use or by indirectly 

 interfering with it. Here we merely draw attention to the extent to 

 which man alters the condition and economic status of the original plant- 

 communities by ameliorating the soil, also by tending cultivated plants 

 and domestic animals, and further point out how, by introducing new 

 cultivated plants (such as forest-trees) and new weeds, he voluntarily 

 or involuntarily brings in fresh forms to compete with native plants. 

 Old plant-communities are eradicated by man, and new ones inaugurated ; 

 for instance, when we see in South America an abandoned plantation 

 filled with weeds in the form of bushes, this is a new, secondary, commu- 



