114 GENETICS AND EUGENICS 



mere act of crossing may reawaken slumbering ancestral 

 traits. Thus he noticed that when rabbits of various sorts 

 are turned loose in a warren together, they tend to revert to 

 the gray-coated condition of wild rabbits. And when pigeons 

 are crossed in captivity they frequently revert to the plum- 

 age condition of the wild rock pigeon, Columha livia. In 

 plants, too, Darwin recognized that crossing is a frequent 

 cause of reversion. The explanation which he gave was the 

 best that the knowledge of his time afforded, but it leaves 

 much to be desired. This lack, however, has been completely 

 supplied by the Mendelian principles. An illustration or two 

 may now be cited. 



When pure-bred black guinea-pigs are mated with red ones, 

 only black offspring are as a rule obtained. (See Fig. 52.) 

 The hairs of the offspring do indeed contain some red pig- 

 ment, but the black pigment is so much darker that it largely 

 obscures the red. In other words, black behaves as an ordi- 

 nary Mendelian dominant. In the next generation black and 

 red segregate in ordinary Mendelian fashion, and the young 

 produced are in the usual proportions, three black to one red, 

 or 1:1 in back-crosses of the heterozygous black with red. 

 All black races behave alike in crosses with the same red 

 individual, but among red animals individual differences 

 exist. Some, instead of behaving like Mendelian recessives, 

 produce in crosses with a black race a third apparently new 

 condition, but in reality a very old one, the agouti type of 

 coat found in all wild guinea-pigs, as well as in wild rats, mice, 

 squirrels, and other rodents. In this type of coat reddish 

 yellow pigment alone is found in a conspicuous band near the 

 tip of each hair, while the rest of the hair bears black pig- 

 ment. The result is a brownish or grayish ticked or grizzled 

 coat, inconspicuous, and hence protective in many natural 

 situations. (See Fig. 53.) 



Some red individuals produce the reversion in half of their 

 young by black mates, some in all, and others, as we have 

 seen, in none, this last condition being the commonest of the 

 three. It is evident that the reversion is due to the intro- 



