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. 



