116 PLANT-LIFE ON LAND [ch. 



they are less obvious they are no less potent in 

 limiting- a plant-population. I mean the inter-rela- 

 tions of organisms. Relatively few plants complete 

 their whole life-cycle independently of other living 

 things, and the degree of inter-dependence is often 

 a surprise to beginners. Darwin in The Origin of 

 S^Mcies cited the example of Red Clover and Cats. 

 Having shown that the Clover is absolutely dependent 

 upon Humble-bees for its pollination, without which 

 no seed is set, he further pointed out that the number 

 of Humble-bees depends in great measure upon the 

 number of Field Mice in the district, which destroy 

 their combs and nests. The number of Mice is 

 dependent on the number of Cats, and he concludes 

 that " 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." 

 Again, it has been shown by elaborate observations 

 in Europe and Asia that the northern limit of dis- 

 tribution of the Monkshood and that of the Humble- 

 bees which pollinate their flowers, is almost exactly 

 coincident, a fact which clearly points to an inter- 

 dependence. Many similar instances might be quoted. 

 But such relations are not confined to the higher 

 plants : Goebel quotes the case of the Splachnaceae, 

 those bright red and yellow Mosses which inhabit the 

 dung and remains of animals. The question is how 



