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 
