38 BULLETIN 1090, TJ. S. DEPARTMENT OE AGRICULTURE. 



with, albinos (c a c a ), it at once becomes apparent that Families 32 

 and 38 alike have the intensity factor (C) , the crossbred young (Cc a ) 

 showing a red spotting which is less intense than that of Family 32 

 but more intense than in Family 38. Families 18 and 35, on the 

 other hand, are proved unmistakably to possess an intermediate 

 factor in the albino series (c k ) the young in both cases having the 

 cream-colored spots typical of heterozygous dilutes (c k c a ). 



In quantity of white spotting, the families have also become 

 markedly differentiated from each other. At one extreme are Fam- 

 ilies 38 and 39 with only about 20 per cent white on the average, and 

 at the other are Families 13 and 31, with about 90 per cent of white. 

 The remaining families are scattered between these limits. Simi- 

 larly, grades of yellow spotting have also become fixed. This sub- 

 ject will be treated in detail in another paper. Here it is enough to 

 note that the 23 families, most of them descended from the same 

 line-bred stock and the rest of them half descended from it, have auto- 

 matically become so differentiated from each other in kind of color, 

 in intensity of color, and in pattern that a new litter could usually 

 be recognized at a glance as belonging to its particular family. 



ABNORMALITIES. 



Another kind of variation in which differentiation among the fam- 

 ilies is clearly shown is polydactylism. Guinea pigs normally have 

 only three toes on the hind feet. It is not uncommon, however, to 

 find a vestigial fourth toe hanging loosely beside the others. Castle 4 

 has shown that this condition is hereditary although not following 

 any simple Mendelian scheme. He produced by selection a stock in 

 which the fourth toe was regularly as well developed as the other 

 toes. This abnormal fourth toe, it may be noted in passing, is inter- 

 esting from an evolutionary standpoint as a true vestigial organ, 

 representing a toe which is present in most rodents but which cavies 

 have nearly lost. It is of a different kind from the extra toes occa- 

 sionally found in cats and man, which are due to a symmetrical redu- 

 plication. The extra toe of guinea pigs is not placed symmetrically 

 with respect to any of the other toes. 



Extra toes were occasionally to be foimd in the stock from which 

 the inbreeding experiments were started. Four cases were recorded 

 in the control stock (B) between 1911 and 1915. Among the inbred 

 families, 1906 to 1915, the distribution of extra toes has been irregular. 

 There were 181 cases recorded in Family 35, 152 in Family 31, 59 

 in Family 38, 26 in Family 36, 25 in Family 11, and 19 in Family 24, 

 while 12 were scattered among Families 2, 7, 14, 17, and 39. None 

 were recorded in Families 1, 3, 13, 15, 18, 19, 20, 21, 23, 32, and 34. 



< Castle, W. E. 1906. The origin of a polydactylous race of guinea pigs. Carnegie Inst. Wash. Pub, 

 No. 49. 



