HUNGER AND LOVE. 261 
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 
