56 THE MEANING OF EVOLUTION 



normal ratio. But the robin is a comparatively slow 

 producer. 



Our turtles are more prolific. Twenty eggs would 

 probably not be an unusual number. If we could im- 

 agine a turtle to live in the sea and to produce at this 

 rate; and, if each turtle should need as much room 

 each way as the robin, and a depth of water equal 

 to its width, before the robins had spread over New 

 York and Pennsylvania the turtles would have filled 

 all the seas of the globe. Frogs are even more re- 

 markable in this respect. Two hundred eggs is not 

 an uncommon number. If each frog required a space 

 twenty-five feet square on which to subsist, the entire 

 earth would be more than covered with them within 

 six years. It is ludicrous to think of such numbers, 

 especially when we realize the hundreds of thousands 

 of kinds of animals there are in the world, each of 

 which is also multiplying, and it becomes evident at 

 once that only an infinitely small proportion of all 

 these creatures can possibly survive. This, then, is 

 multiplication. 



Here comes into play the fourth basal idea in Mr. 

 Darwin's explanation. This is the part of Selection. 

 When man produces new varieties of animals he does 

 it by picking out from his flocks or his herds such as 

 conform most nearly to his idea of what is desirable. 

 These he mates, and from their progeny he selects 



