CHAPTER V 
THE PROCESS OF CHANGE FROM A REPTILE TO A 
BIRD 
WE have now decided that birds have sprung from 
some reptilian stock, though not from any existing 
order of reptiles, and unless we are prepared to differ 
from the great majority of biologists, we must hold that 
this has been brought about mainly by the struggle for 
existence. All animals multiply rapidly, and, if there 
were no check, this would continue in geometrical 
ratio, till there would be enough of a single species to 
people all the earth. Thus, if one pair left two pairs 
of young, these two would leave four pairs, and 
those four eight, and so forth. Linnzus calculated 
that an annual plant producing two seeds in a year 
would after twenty years have a million descendants, 
And as every plant produces many more than two 
seeds—a horse-chestnut tree, for instance, many thou- 
sands—every real instance would be far more telling 
than this imaginary one. The elephant has very few 
young. Darwin’s estimate allows it six in all, born 
while it is between the ages of thirty and ninety, 
