542 



THE CHANGING WORLD OF LIFE 



cause they are not passed on to the offspring. Through the rigorous 

 method of natural selection, however, there is an adaptation of the 

 species, and these adaptations are inherited. This is the main force of 

 evolution. Briefly, natural selection involves the overproduction of off- 

 spring with the destruction by the forces of nature of those most poorly 

 adapted to the environment. This is possible because of the natural 

 variation that exists among living things due to the variations of the 

 genes, even among offspring of the same parents. It stands to reason 

 that whenever there is a difference among living things it is almost cer- 

 tain that there will be some that will be better fitted for the particular 



Photos by Winchester 



Fig. 34.1. Adaptation to changing seasonal environment. The Willow Ptarmigan 

 has feather changes which make it blend with its environment in the different seasons 



of the year. 



conditions under which these organisms live than others. In the long 

 run, the best adapted will survive to reproduce and transmit their de- 

 sirable genes to the future generations, while the undesirable genes 

 would be eliminated along with the bodies of their carriers. This may 

 seem like a somewhat wasteful and cruel method, but that is the way it 

 is and no living things can escape the relentless force of natural selection. 



The degree of selection varies considerably according to the number 

 of young produced ; those animals that do not give care to their young 

 must make up for it by producing more than those that give a high 

 degree of protection. Since more are destroyed in the first case, the 

 degree of selection is greater when large numbers are produced and left 

 to shift for themselves from the first. 



A mature female bullfrog will lay about 15,000 eggs per year and, 

 on the average, will reproduce about five years. If all of these hatched 



