62 EVOLUTION OF LIVING ORGANISMS. 



The severity of the struggle may vary according to age 

 and the surrounding conditions, but it is never quite 

 absent. Besides the competition for food, for light, for 

 water, for space generally, and the unceasing struggle 

 against unfavourable conditions of climate, there is the 

 never-ending war against enemies, parasites, and dis- 

 eases. All these factors contribute towards the death- 

 rate of the species. 



How great are the possibilities of increase and how 

 much they are kept in check under ordinary conditions, 

 is seen when the balance of factors restraining it is in 

 any way disturbed, as for instance owing to some change 

 in the climate or the intervention of man. The occasional 

 immense swarms of locufets, of butterflies or other insects, 

 of lemmings, the great epidemics of diseases, are ex- 

 amples of the temporary removal of barriers to repro- 

 duction. Similar expansion of some one species at the 

 expense of others is continually taking place on a small 

 scale even in the most stable fauna and flora. The ex- 

 traordinarily rapid increase of horses and cattle intro- 

 duced by the Spaniards in South America, of rabbits in 

 Australia, of terrible insect pests like the Phylloxera 

 which attacked the vines in Europe, or the European 

 gipsy moth (Lymantria dispar) which destroys the forest 

 trees in North America, the spread of the watercress 

 (Nasturtium officinale) blocking the rivers of New 

 Zealand, or the American water- weed (Elodea canadensis) 

 filling the rivers of western Europe, these are only a few 

 of hundreds of similar cases of the possible result of in- 

 troducing a species into a new country where it does not 

 meet with the ordinary checks occurring in its native 

 habitat. 



Individuals of similar habits stand most in each other's 

 way ; therefore, competition is often more severe between 

 closely allied than distantly related forms. It is most 

 severe within the limits of the species itself, between 

 individuals, families, tribes, and social aggregations. 



