178 



Factors 

 which in- 

 fluence the 

 Relations 

 existing 

 between 

 Organisms 

 in Nature. 



probable that, if the whole genus of humble-bees became 

 extinct or very rare in England, the * red clover would 

 become very rare, or wholly disappear. The number of 

 humble-bees in any district depends in a great measure upon 

 the number of field-mice, which destroy their combs and 

 nests. * * Now the number of mice is largely de- 



pendent, as everyone knows, on the number of cats. * 

 Hence it is quite credible that the presence of a feline 

 animal in large numbers in a district might determine, through 

 the intervention first of mice and then of bees, the frequency 

 of certain flowers in that district ! " * We frequently do not 

 realize the severity of this great struggle for in nature a bal- 

 ance is generally maintained between the conflicting forces, 

 a position of more or less stable equilibrium being eventually 

 arrived at, which, however, a very trifling circumstance may 

 entirely upset. The plant known as Lantana aculeata is a 

 native of America and was introduced into Ceylon about 1824. 

 Since then it has spread with extraordinary rapidity over the 

 Peninsula of India, where it " now covers, with a dense network 

 of intertwined branches, large areas of country, almost to 

 the complete exclusion of other vegetation." f This well 

 illustrates how a plant, the development of which is favoured 

 more than is that of other species by the factors of the locality, 

 soon succeeds in eliminating all other vegetation. Again 

 man, by forming pure forests of a single species of tree, has 

 favoured the development of the insects and fungi which prey 

 upon it, by collecting together and making easily accessible 

 for them masses of their favourite food, a procedure which 

 more than once has led to the total destruction of extensive 

 forests in Europe, while in the teak forests of India we clearly 

 see how much more damage is done in pure forests than in 

 mixed by defoliators, such as Hyblae:i puera and Pyrausta 

 machoeralis. 



156. Let us now try to realize 



the factors which induce one organism to injure or help 

 another and which enable it to do so, as well as the conditions 

 which enable life to still continue on the earth amidst so much 

 destruction. 



Complex carbon compounds generally known as organic 

 materials form an essential part of the food of all living 

 organisms. Green plants alone among all living organisms 



* Origin of Species. By Charles Darwin, 6th Ed., pages 53 54. 

 t A Manual of Indian Timbers. By J. S. Gamble, 1902, page 524. 



