ORIGIN OF THE LIVING MACHINE: ADAPTATION 361 



spring will show the dominant character, while one-fourth 

 will show the recessive character. If now the individuals show- 

 ing this recessive character are bred with each other, all their 

 offspring will show the recessive character, the dominant char- 

 acter having totally disappeared from them, never to occur 

 again in any subsequent generation. This race is then a pure 

 recessive type, from which all of the dominant characteristics 

 have been eliminated. All of the other three-fourths of the 

 second generation show the dominant character only. But 

 tests, similar to the above, prove that only one of these is 

 purely dominant. The other two-fourths, although in them 

 the dominant character only is evident, are really mixed, con- 

 taining both dominant and recessive characters. This is shown 

 by the fact that if they are crossed, three-fourths of their off- 

 spring will again show the dominant character and one-fourth 

 will show the recessive character. This process may then be re- 

 peated indefinitely. 



An illustration may make this clearer. Among mice the color 

 gray is dominant, while the color white is recessive. If white 

 and gray mice are bred together, the first generation of offspring 

 will be all gray. If these gray animals are now bred together, 

 in the second generation three-fourths of the offspring will be 

 gray but one fourth will be white. If these white animals are 

 bred together, their offspring will all be white and will continue 

 to breed white offspring indefinitely, no gray mice ever subse- 

 quently appearing in their progeny. They constitute a pure 

 white race. If the other three-fourths, which are gray, are bred 

 together, one of the three-fourths will continue indefinitely to 

 produce gray offspring, no white ones appearing. In these the 

 white characteristic has been eliminated entirely, and they form 

 a pure gray race. But the other two-fourths, when bred to- 

 gether, prove to contain both white and gray characters, and 

 among their offspring one-fourth will show the white fur and the 

 other three-fourths the gray fur. If again tested in the same 

 way, the white animals will be found to produce pure races of 



