THE UNDERLYING IDEA 55 
miliar story of the blacksmith who asked the price at 
which the stranger would sell the horse he was shoe- 
ing. The owner of the horse replied that, if the black- 
smith would give him one penny for the first nail he 
drove into the shoe, two for the second, four for the 
third, and so on, he might have the horse. No hun- 
dred horses in the world taken together have ever 
brought such a price as the blacksmith would have 
had to pay for the animal on which he was working. 
This is no circumstance to the awful story of what 
would happen to the earth if any animal could multi- 
ply unrestricted. The usual number of eggs laid by 
a mother robin for a single brood is four, and she may 
produce two broods in one season. This would mean 
that the original pair had produced eight offspring, 
four times their own number. If we can imagine 
these mating the next year and producing their kind 
in the same proportion; and, if we further suppose 
that each robin needs a space one hundred feet square 
from which to gather his food, we realize the aston- 
ishing fact that in fifteen years every patch one hun- 
dred feet square in Pennsylvania and New York would 
each have its resident robin, while the following sea- 
son would find a robin on every similar patch from 
Maine to the Carolinas. Of course this could never 
happen, this is simply what would happen if all the 
robins could grow to maturity and reproduce at the 
