50 THE THEORIES OF EVOLUTION 



what determines the outcome of that competitive 

 fight? Slight individual differences, insignificant 

 characteristics which enable those possessing them to 

 overcome their opponents. 



"Let us take," Darwin writes, "the case of a wolf 

 which preys on certain animals, securing some by- 

 craft, some by strength and some by fleetness ; and let 

 us suppose that one species of animals, deer for in- 

 stance, had from any change in the country increased 

 in numbers, or that other prey had decreased in num- 

 bers, during that season of the year when the wolf 

 was hardest pressed for food. The survival of the 

 wolves would then depend upon their ability to catch 

 deer, the fleetest of the animals on which they can 

 prey. Under such circumstances the swiftest and 

 slimmest wolves have the best chance of surviving 

 and of leaving descendants which inherit their fleet- 

 ness." 



Another example is taken from the vegetable king- 

 dom: Certain plants excrete sweet juice, greedily 

 sought by insects; this is effected, for instance, by 

 glands at the base of the stipules or at the back of the 

 leaves. Now let us suppose that the juice or nectar 

 were excreted from the inside of the flowers instead 

 of from the back of the leaves: insects in seeking 

 the nectar would get dusted with pollen and trans- 

 port it from one flower to another. The flowers 

 of two distinct individuals of the same species would 



