THE UNDERLYING IDEA 49 
The second great underlying idea is known by the 
name of Variation. We have just been dwelling on 
the regularity with which parents produce offspring 
like themselves. We must now draw attention to the 
fact that, while it is true animals must absolutely be- 
long to the same genus or species, even to the same 
variety, none the less no animal is exactly like his 
parents. Furthermore, in a group of animals pro- 
duced at the same time from the same parent each 
one will have at least some small point in which he 
differs from every other one in the group. Two ani- 
mals may look alike at first to the undiscerning eye, 
but a keen analysis of the measurements of the vari- 
ous parts of their bodies will show distinct differ- 
ences. This is quite as true among lower animals. 
A toad may lay a double string of four hundred eggs 
which may be fertilized by the same male at the same 
time. These eggs may develop into tadpoles in the 
same pool not over a foot square. Within a few 
weeks these little toads may have gained their legs, 
lost their tails, and all may have left the water and 
taken to the ground upon the same day. Already the 
careful observer will notice differences among them. 
Some are larger than others, having grown more rap- 
idly even though their surroundings were exactly the 
same; others are more skillful in their peculiar method 
of throwing the tongue at an insect they wish to catch. 
