DAIiWlX. r.EFORE AND AFTER 105 



doiiiostic pigeons. Tliere are many races and varieties, 

 wliich dilfer in sliape, colour, size and instincts. For 

 example, there are the carrier pigeons, with their special 

 toj)ographical instinct; the tumblers, which are in the 

 habit of flying at a great height in compact flocks, and 

 then tumbling in the! air head over heels; the fantails, 

 whose tail, owing to the disposition of its feathers, re- 

 .sembles that of a peacock; others have tufts of feathers, 

 peculiar creases of the skin, modified beaks or feet, or 

 other marked features. Darwin secured specimens of 

 every available breed from all parts of the world. He 

 became acquainted with the best known breeders and 

 fanciers, joined two London pigeon clubs, and after years 

 of observation, succeeded in pi'oving that all the various 

 breeds were descended from one single species, the 

 European rock pigeon, CoUimha livia. Previously, 

 breeders had believed the difl'erent varieties to be derived 

 from as many wild species. 



Again, Darwin discussed the breeding of the elephant. 

 The elephant is considered to be the slowest breeder 

 known. Yet if one elephant produces six young, then 

 after a period of 750 years, there would be about nineteen 

 million elephants alive, all descended from the first pair. 

 If all species of animals increased at the same ratio, 

 their numbers would become so inordinateh' great that 

 no counti'y could support them. 



In 1858 he wrote : — '' Even slow-breeding mankind has 

 doubled in twenty-five years; and if he could increase 

 his food with greater ease, he would double in less time. 

 But for animals without artificial means, the amount of 

 food for each species must, on an average, be constant,- 

 whereas the increase of all organisms tends to be geo- 

 metrical, and in a vast majority of cases at an enormous 

 ratio. Suppose in a certain spot there are eight pairs 

 of birds, and that only four pairs of them annually (in- 

 cluding double hatches) rear only four young, and that 

 these go on rearing their young at the same rate, then 



