THE ORIGIN OF SPECIES AND GENERA 301 



which lead to the selection of new forms, and the enormous 

 selecting power brought to bear owing to the rapid 

 increase and corresponding great annual mortality among 

 all animals, it is impossible to doubt that the means are 

 adequate to the result. To bring these means clearly 

 before our readers, let us suppose that a pair of birds 

 produce every year six young, and that they live for five 

 years. We thus have thirty birds out of which to replace 

 the two, so that, on the average, at least twenty-eight 

 must die during this time, and many more if any of these 

 live to breed along with their parents. This gives us, as 

 a minimum, a destruction every five years of fourteen 

 times as many birds as exist at any one time. Now let 

 us suppose a change going on Avhich renders it beneficial 

 for a species to obtain longer wings in order to escape 

 from some enemy, and a stronger bill to enable it to 

 capture some fresh insect, both of which (the enemy and 

 the insect) are gradually increasing in the country. Varia- 

 tions of both these kinds occur in abundance every year, 

 to an amount measured by ten or twenty per cent, of the 

 average dimensions. Either of the variations would be 

 useful and would be preserved separately, Avhile the com- 

 bined variation would be doubly useful and would also be 

 preserved whenever it appeared. A race in which these 

 two characters were from ten to twenty per cent, above 

 the average would therefore be easily produced in twenty 

 or fifty years ; while in a thousand or five thousand years 

 a change amounting to thirty or forty per cent. — far 

 greater than distinguishes many species — would probably 

 be brought about. This illustration, I think, renders it 

 clear that the extreme slowness of the action of natural 

 selection on which Mr. Darwin repeatedly dwells, is by no 

 means an essential characteristic of it, but is only due to the 

 fact that physical and other conditions usually change wdth 

 extreme slowness. But if, as must often have happened, 

 conditions have changed with comparative rapidity, then 

 the enormous amount of individual variation, which would 

 be taken advantage of every year by the survival of the 

 fittest, might effect changes in a single century quite as 

 great as those which distinguish nearly allied species. 



