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. Linnaeus 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, 



