CH. X.] NATUBAL SELECTION. 16 



accumulating on either side of the mean which is most 

 advantageous to the species. When the environment changes, 

 if certain variations on one side of the established mean 

 tend to bring the individuals which manifest them into closer 

 adaptation to the new environment, these individuals will 

 survive in the struggle for life, and thus the average character 

 of the species will be slightly altered. No two bears have 

 just the same amount of hair, no two moths have just the 

 same length of proboscis, no two antelopes are exactly matched 

 in fleetness. Now if increasing cold renders a thicker 

 covering useful to the bear, or if the lengthening of a flower- 

 calyx, due to a slight change in soil or quantity of sunlight, 

 renders a longer proboscis useful to the moth, or if the 

 immigration of a carnivorous animal makes it necessary for 

 antelopes often to run for their lives, then in each generation 

 the thickest-coated bears, the longest-tongued moths, and the 

 fleetest antelopes wall survive. Every individual variation 

 in the direction of a heavier coat, a longer sucker, or a 

 structure better adapted for fleeing will give its owner the 

 advantage in the incessant struggle for life, and these 

 peculiarities will be oftenest inherited, while individuals 

 which do not vary, or which vary in the wrong direction, 

 will have to migrate or die.^ 



The student of natural history, who realizes, however 

 dimly, the prodigious complexity of the relations of the 

 various species of animals and plants to each other, will 

 Derceive that the amount of variation thus preserved and 

 enhanced must in the course of long ages become enormous. 

 If a grain of sand were each year added to an ant-heap, it 

 would in course of time become as large as Chimborazo. But 

 these changes, directly caused by natural selection, are greatly 



* It is thus one of the great merits of the theory of natural selection, that 

 It accounts for the phenomena of extinction of species, — wliich formerly could 

 only be accounted lor by the gratuitous and utterly indefensible hypothesis 

 of periodical catastrophes or cataclysms. 



