THE CHANGING WORLD OF LIFE 



543 



and grew to adult size the earth would be covered with bullfrogs in 

 several generations. Most of the eggs hatch, but the tadpoles and young 

 frogs furnish food for fish, snakes, birds, mammals, and even frogs, 

 including their father and mother. On the average, about two out of 

 the entire group will survive to reach the adult stage and reproduce. 

 These two, however, will be a highly selected pair; there has been no 

 chance elimination that has left these two alone from the great host of 



Photo by Winchester 



Fig. 34.2. One out of the 37,500. This puffed up bullfrog has good reason to be 

 proud. He alone has survived while about 37,499 of his brothers and sisters perished 

 in the rigorous process of natural selection. In order to have survived this great 

 slaughter, this frog must have many desirable characteristics which he can pass on 



to future generations. 



their brothers and sisters. A fish catching a tadpole would catch the 

 one that swam the slowest, that was most conspicuously colored, that 

 was the slowest to detect approaching danger. This would leave the fast 

 swimmers, the protectively colored, and the alert tadpoles alive to con- 

 tinue the struggle for existence. When food became scarce, those tad- 

 poles that could detect its presence and get to it first lived, while their 

 less sensitive and aggressive brothers and sisters starved. When they 

 changed into young frogs and crawled up out of the water, those that 

 learned to jump first reached the safety of the water, leaving the slower 

 and more curious behind to be eaten by birds. Then, there are fungus 



