HUNGER AND LOVE. 26 1 



of the palm, and consequently of man; birds, mites, and 

 insect-eating birds would, on the other hand, endanger it. 



Interesting examples in relation to the change of correla- 

 tions in the struggle for life are furnished also by those 

 isolated oceanic islands, uninhabited by man, on which at 

 different times goats and pigs have been placed by 

 navigators. These animals become wild, and having no 

 enemies, they increase in number so excessively, that the 

 rest of the animal and vegetable population suffer in conse- 

 quence, and the island finally may become almost a waste, 

 because there is insufficient food for the large mammals 

 which increase too numerously. In some cases on an island 

 thus overrun with goats and pigs, other navigators have let 

 loose a couple of dogs, who enjoyed this superabundance of 

 food, and they again increased so numerously, and made 

 such havoc among the herds, that after several years the dogs 

 themselves lacked food, and they also almost died out. The 

 equilibrium of species continually changes in this manner in 

 nature's economy, accordingly as one or another species 

 increases at the expense of the rest. In most cases the 

 relations of different species of animals and plants to one 

 another are much too complicated for us to be able to follow 

 them, and I leave it to the reader to picture to himself what 

 an infinitely complicated machinery is at work in every part 

 of the world in consequence of this struggle. The impulses 

 which started the struggle, and which altered and modified 

 it in different places, are in the end seen to be the impulses 

 of self-preservation — in fact, the instinct leading individuals 

 to preserve themselves (the instinct of obtaining food), and 

 the instinct leading them to preserve the species (instinct of 

 propagation). It is these two fundamental instincts of 



