150 THE MAIN CURRENTS OF ZOOLOGY 



idea. A single trout, for illustration, lays from 

 60,000 to 100,000 eggs. If the majority of these 

 arrived at maturity and gave rise to progeny, the 

 next generation would present a prodigious number, 

 and the numbers in succeeding generations would 

 increase so rapidly, that soon there would not be 

 room in the fresh waters of the earth to contain their 

 descendants. It is true that most animals produce 

 fewer eggs (some lay more), but observation estab- 

 lishes the truth that animals tend to multiply by 

 geometric progression, while as a matter of fact the 

 number of any one kind remain practically constant. 



The agency that determines which organisms shall 

 be preserved in the struggle for existence is natural 

 selection. This agency will favor certain variations 

 in structure that may prove of advantage, so that, 

 in the long run, those best adapted to conditions of 

 life would survive. 



Natural selection works in such a variety of ways 

 that a few illustrations are essential. Fleetness in 

 such animals as antelopes may be the particular 

 thing which secures their safety. In all kinds of 

 strain due to scarcity of food, inclemency of weather 

 and other rigorous conditions, those forms with the 

 best powers and physiological resistance will have 



