to form Varieties* 



6301 



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 thai the animal population of the 

 globe must be stationary, or perhaps, through the influence of man, 

 decreasing. 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 in- 

 creasing 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 sup- 

 pose 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 

 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 immense 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, what- 

 ever be the average number of individuals existing in any given 

 country, twice that number must perish annually, — a striking result, 

 but one which seems at least highly probable, and is perhaps under 

 rather than over the truth. It would therefore appear that, as far as 

 the continuance of the species and the keeping up the average num- 

 ber of individuals are concerned, large broods are superfluous. On 

 the average all above one become food for hawks and kites, wild cats 

 and weasels, or perish of cold and hunger as winter comes on. This 

 is strikingly proved by the case of particular species ; for we find that 

 their abundance in individuals bears no relation whatever to their 

 fertility in producing offspring. Perhaps the most remarkable instance 

 of an immense bird population is that of the passenger pigeon of the 

 United States, which lays only one, or at most two eggs, and is said 

 to rear generally but one young one. Why is this bird so extra- 

 ordinarily abundant, while others producing two or three times as 



