THE STRUGGLE FOB EXISTENCE. 67 



Darwin, Wallace, and others. Every observant naturalist 

 now realises how intense is the struggle, how rapidly 

 one species can be replaced by another, how completely 

 the geographical distribution, the abundance or rarity, 

 or the extermination of an organism is dependent on the 

 success of its competitors. It is the selective action of 

 this competition, combined with the selective action of 

 the inorganic factors of the environment, which regulates 

 the distribution of living things and the diversity in the 

 fauna and flora found in different regions. If this selec- 

 tive action could be stopped, the whole fauna and flora 

 would soon become almost uniform throughout the 

 world. 



The well-known instance o cats and clover, quoted by 

 Darwin, well illustrates the extraordinarily complex 

 interrelationship between different organisms of most 

 diverse structure and most distant affinity. Bed clover 

 (Trifolium prat&nse) is almost exclusively fertilised by 

 humble-bees, and the abundance of bees in a district is 

 much influenced by the number of mice, which destroy 

 their nests and eggs. Now since the number of mice 

 depends to a great extent on the abundance of cats, these 

 animals will affect the distribution of the red clover in the 

 neighbourhood. So the goats introduced into St. Helena, 

 by eating up the seedling trees, not only destroyed the 

 forests, and greatly altered the general flora and fauna of 

 the island, but have actually changed the climate itself. 

 The notorious tse-tse fly (Glossina morsitans\ carrier of 

 the deadly disease germ (Trypanosoma Brucei), renders 

 certain districts of South Africa uninhabitable for im- 

 ported dogs, horses, and cattle ; while the distribution 

 of man himself has been greatly influenced by the mos- 

 quito Anopheles, and the malaria parasite which it trans- 

 mits. But the continuous steady struggle which goes on 

 between more closely allied forms is perhaps best seen in 

 the case of plants. It can be watched in the forest and 

 on the mountain slope, in the desert and in the tropical 



