FACTORS CONDUCIVE TO DISEASE 77 



ing the cocoa tree, for the purpose of this work, may be 

 considered as the variation from the normal of functions 

 which threaten the life of the tree, and this implies danger 

 of premature death. 



In maintaining the plant in a good state of health by 

 a careful observance of its requirements in regard to soil, 

 light, and air, the planter is adopting the best preventive 

 measures against disease. 



Serious epidemics of disease rarely occur amongst 

 plants growing in a wild state. This is mainly due to 

 the fact that a specific disease almost invariably con- 

 fines its attacks to nearly allied species. Under natural 

 conditions, large numbers of unrelated plants grow 

 intermixed, and it very rarely happens that a single 

 species of plant monopolises a large area of land. Any 

 particular diseased plant is more or less isolated and the 

 spread of the disease to plants of a similar species is 

 thus checked, as the plants in the immediate vicinity 

 are unrelated and are consequently more or less immune 

 from this particular disease. 



The practice of cultivating large areas of land with 

 a single species of plant is therefore most conducive to 

 the diffusion of the various diseases to which the plant 

 is subject. In the wild state there is a constant struggle 

 for existence, the plants least adapted to grow in any 

 particular situation are crowded out by those better 

 adapted, which results in the survival of the fittest. 

 The very fact of a plant thriving in any particular dis- 

 trict in a wild state indicates that the natural conditions 

 are favourable for its development. 



When this plant is introduced to a new country and 

 is grown under the conditions which cultivation involves, 

 some of the factors which might be inimical to its 

 growth in a natural state are obviously removed, but it 

 is introduced to others to which it was not previously 

 subjected. In all probability it will be still affected 

 by the various diseases which it encountered in its 

 native habitat, but many of the factors which previously 

 kept these in check will be absent, and an endemic disease 

 may then develop into an epidemic. For example, 

 the depredations of an insect, which fed upon its tissues, 

 may have been held in check by being preyed upon by 

 another in its native home, but the former may be present 



