6. THE ORIGIN OF A POLYDACTYLOUS RACE OF GUINEA-PIGS. 



BY W. E. CASTLE. 



I. FIRST APPEARANCE OF THE POLYDACTYL CHARACTER. 



Normal guinea-pigs have four toes on each front foot but only three 

 on each hind foot. The race of guinea-pigs whose origin is to be described 

 differs from ordinary guinea-pigs in possessing four digits on each hind 

 foot instead of three. The four digits of the front foot apparently corre- 

 spond with digits 2-5 of the typical five-fingered appendage, while the three 

 digits of the hind foot correspond with digits 2-4 of the typical appendage. 

 In other words, all guinea-pigs have lost from the front foot the digit which 

 corresponds with our thumb, and normal guinea-pigs have lost from the hind 

 foot two digits, which correspond respectively with our " great " and 

 " small " toes. In the race to be described the " small toe " is present on 

 the hind foot as well as on the front. 



In October, 1900, several ordinary guinea-pigs were obtained from a 

 nearby breeder, one pair of which produced in the following June three 

 male young. The largest of these young bore an imperfectly developed 

 digit on its left hind foot, in the position of a " small toe." This toe bore 

 a well-developed claw and apparently contained the bony phalanges, but 

 these evidently were not joined with the foot by appropriate muscular and 

 tendinous connections, for the toe hung limply down from the side of the 

 foot like a bag of skin. It remained attached to the foot until the animal 

 was fully grown, but was then lost, probably having been accidentally 

 pulled off. 



Previous to the birth of this polydactylous guinea-pig, I had never seen 

 a fourth digit on the hind foot of a guinea-pig, nor heard of its occurrence 

 either among wild or domesticated Caviidae, and I am unable to find any 

 reference to such a character in the literature of the group. But I have 

 since found that the " extra-toe " does occur not infrequently among guinea- 

 pigs in an imperfectly developed condition, and I have twice since obtained 

 animals of this sort from breeders. From the progeny of the single poly- 

 dactylous individual born in June, 1901, a well-established race of four-toed 

 guinea-pigs has now been formed. An account of how this was done, it 

 is hoped, may be of value as bearing on the origin of breeds. 



