54 THE STRUCTURE AND LIFE OF BIRDS chap. 



its term of life being estimated at one hundred years. 

 Yet with these data he calculates that " after a period 

 of from 740 to 750 years there would be nearly 

 19,000,000 elephants alive, descended from the first 

 pair." 



This marvellously rapid increase of all species was one 

 of the two cardinal facts on which his theory of the 

 origin of species was based. The other was the constant 

 tendency to variation. The progeny are very like, 

 but never exactly like their parents. He took his 

 instances mainly from the domesticated animals, 

 because sufficient evidence had not then been collected 

 from wild nature. All the domestic pigeons — the 

 Fantail, the Pouter, the Dragon, the Carrier, the 

 Homer, the Runt, etc. — had been derived from one 

 wild stock, the Rock Dove. The breeder had per- 

 formed equal wonders with cattle and horses, and 

 during the many thousands of years that the world had 

 been peopled with animals and plants, nature had been 

 doing what the breeder had begun to do only some 

 centuries ago. She had been constantly weeding out 

 those that were less fitted to live. The rocks bear 

 records of thousands of extinct species that have made 

 room for others. Darwin, as I have said, assumed that 

 variation occurred in wild species as among domestic 

 animals. But until this assumption had been proved 

 true, clearly the theory rested upon an insecure 

 foundation. Many observations have now been made, 

 and any one who wishes for a detailed account of them 

 may find it in Dr. Russel Wallace's Darwinism. 

 He shows conclusively that in wild species there are 

 two principles working side by side : the principles of 



