10 COSMFC PHILOSOPHY. [pt. ii. 



blood-liound are more tlian ten times as large as 1,lie terrier, 

 and, if found in a state of nature, they would perhaps be 

 classed in distinct genera, like the leopard and panther, whose 

 differences are hardly more striking. Yet the ancestral races 

 from which these dogs have been reared differed but slightly 

 from each other. The different breeds of dogs vary in the 

 number of their toes, teeth, and vertebrae, in the number 

 and disposition of their mammae, in the shape of their 

 zygomatic arches, and in the position of their occiputs; 

 although dogs have not been selected with reference to these 

 peculiarities, about which uninstructed men neither know 

 nor care, but only with reference to their speed, fleetness, 

 strength, or sagacity. In the case of domestic pigeons, where 

 man has been to a great extent actuated by pure fancy in his 

 selections, the divergences are still more remarkable. All 

 domestic pigeons are descended from a single species of wild 

 pigeon ; yet their differences, even in bony structure, in the 

 internal organs, and in mental disposition, are such as charac- 

 terize distinct genera, and to describe them completely would 

 require a large volume. Pigs, rabbits, cows, fowl, silk- moths, 

 and hive-bees furnish no less instnictive evidence ; and the 

 development of the peach and the almond from a common 

 stock, and of countless varieties of apple from the sour crab, 

 may be cited, out of a hundred examples, to show what pro- 

 digies artificial selection has accomplished in the modification 

 of vegetal organisms. 



ISTow Mr. Darwin's great achievement has been to show 

 that a similar process of selection, going on throughout the 

 organic world without the knowledge or intervention of 

 man, tends not only to maintain but to produce adaptive 

 alterations in plants and animals. The process is a simple 

 one, when once we have the clew to it. All plants and 

 animals tend to increase in a high geometrical ratio. The 

 old problem of the nails in the horse's shoe teaches 

 us what an astounding affair is a geometrical rate of in- 



