14 POPULAR SCIENCE MONTHLY. 



The life of wild animals is a struggle for existence. The full ex- 

 ertion 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 

 anitnals 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, 

 decreasing. Fluctuations there may be; but permanent increase, ex- 

 cept 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 power- 

 ful 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 ratio 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 to nearly ten millions! 

 whereas we have no reason to believe that the number of the birds of 

 any country increases at all in fifteen or in one hundred and fifty 

 years. With such powers of increase the population must have reached 

 its limits, and have become stationary, in a very few years after the 

 origin of each species. It is evident, therefore, that each year an im- 

 mense number of birds must perish — as many in fact as are born; 

 and as on the lowest calculation the progeny are each year twice as 

 numerous as their parents, it follows that, whatever be the average 

 number of individuals existing in any given country, twice that number 



