544 



THE CHANGING WORLD OF LIFE 



diseases that claim a toll of the more susceptible ones, leaving those 

 highly resistant to such infections. 



When the bullfrogs attain sexual maturity three or four years after 

 hatching, they represent a highly select group. The many characteris- 

 tics that these frogs possess that enabled them to survive while their 

 brothers and sisters perished by the thousands all around them are 

 largely hereditary. Therefore, the next generation of bullfrogs will be 

 better able to withstand all these unfavorable environmental conditions. 



These survive , 

 and reproduce 



I 1 \ 



From Genetics, Winchester, Houghton Mifflin 



Fig. 34.3. The concept of natural selection as proposed by Charles Darwin. A pair 

 of African antelopes produce about eight offspring, on the average, before they die. 

 An average of only two of these survive in the struggle for existence. This is re- 

 peated each generation, thus creating a constant selection for those best suited to the 



environment. 



If this is true, it may seem that a greater proportion of them should 

 survive the second generation, still more the third, and so on. It is not 

 this way, however, for it must be remembered that the frogs are not the 

 only animals undergoing natural selection. All the fish, the birds, the 

 reptiles, and even the fungus parasites are developing new and better 

 ways of killing tadpoles and little frogs through this same rigorous 

 process. It is now clearly evident why the statement was made at the 

 first of the chapter that no animals can remain as they are and continue 

 to exist. Their enemies are not standing still ; they are constantly de- 



