54 THE STRUCTURE AND LIFE OF BIRDS cap. 
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 
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