Darwin- Wallace Celebration. 99 



and to give rise to successive variations departing further 

 and further from the original type, and which also produces, 

 in domesticated animals, the tendency of varieties to return 

 to the parent form. 



The life of wild animals is a struggle for existence. The 

 full exertion of all their faculties and all their energies is 

 required to preserve their own existence and provide for that 

 of their infant offspring. The possibility of procuring food 

 during the least favourable seasons, and of escaping the 

 attacks of their most dangerous enemies, are the primary 

 conditions which determine the existence both of individuals 

 and of entire species. These conditions will also determine 

 the population of a species; and by a careful consideration of 

 all the circumstances we may be enabled to comprehend, 

 and in some degree to explain, what at first sight appears so 

 inexplicable the excessive abundance of some species, while 

 others closely allied to them are very rare. 



The general proportion that must obtain between certain 

 groups of animals is readily seen. Large animals cannot be 

 so abundant as small ones ; the carnivora must be less 

 numerous than the herbivora; eagles and lions can never be 

 so plentiful as pigeons and antelopes; the wild asses of the 

 Tartarian deserts cannot equal in numbers the horses of the 

 more luxuriant prairies and pampas of America. The greater 

 or less fecundity of an animal is often considered to be one 

 of the chief causes of its abundance or scarcity ; but a 

 consideration of the facts will show us that it really has little 

 or nothing to do with the matter. Even the least prolific of 

 animals would increase rapidly if unchecked, whereas it is 

 evident that the animal population of the globe must be 

 stationary, or perhaps, through the influence of man, de- 

 creasing. Fluctuations there may be ; but permanent 

 increase, except in restricted localities, is almost impossible. 

 For example, our own observation must convince us that 

 birds do not go on increasing every year in a geometrical 

 ratio, as they would do, were there not some powerful check 

 to their natural increase. Very few birds produce less than 

 two young ones each year, while many have six, eight, or 

 ten ; four will certainly be below the average ; and if we 

 suppose that each pair produce young only four times in 

 their life, that will also be below the average, supposing 

 them not to die either by violence or want of food. Yet at 

 this rate how tremendous would be the increase in a few 

 years from a single pair ! A simple calculation will show 

 that in fifteen years each pair of birds would have increased 



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