PRENTISS: POLYDACTYLISM IN MAN AND DOMESTIC ANLMALS. 301 



qualities peculiar to a distant ancestor, — qualities which were once 

 characteristic of the species, but have been lost in the evolution of 

 varieties. Consequently, the best-authenticated instances of reversion are 

 those in which individuals of a certain variety or breed return to the 

 characters of the original species. Well-known examples are the rever- 

 sion of domestic varieties to the character of the wild rock-pigeon ; the 

 recurrence of shoulder-stripes and a dun coloration in the horse and mule; 

 the appearance of longitudinal stripes on the backs of young domestic 

 swine when allowed to return to the feral state, — a coloration pecu- 

 liar to the sucklings of the wild ancestors of the hog, but normally want- 

 ing in the young of the domestic pig. In these cases, which we know 

 are reversionary, it may be observed (1) that the phenomenon is simply 

 the return of individuals of a variety to the original characteristics 

 of the species ; (2) that the variation in such reversions relates merely 

 to the degree of completeness with which the atavistic qualities are 

 transmitted ; monstrous conditions, or malformations, are never thus 

 produced. 



In animals in which the typical number of functional digits is normally 

 reduced (pes of Carnivora, swine, ruminants, and Equidae), the super- 

 numerary digits in the majority of cases are developed independently of 

 the normal digits, but in connection with embryonic vestiges or rudi- 

 ments. Is not reversion, then, the factor which is operative here, caus- 

 ing the development of degenerate digits, and thus tending to restore 

 the original pentadactyle condition 1 The objection is raised, however, 

 that tliere is too great a disttince in point of time and relationship between 

 the polydactyle animal and the pentadactyle ancestor to which it is sup- 

 posed to revert. According to the old idea of heredity this might seem 

 true, but in the light of Mendel's law (recently fully confirmed) it is no 

 longer a serious objection. As pointed out by Bateson and Saunders (:02) 

 and Castle (:03), the important facts discovered by Mendel are that a 

 single parental character may be segregated in the germ-cells of the off- 

 spring, and that one of a pair of parental characters may regularly domi- 

 nate over the other ; further that each of the offspring, though exhibiting 

 the dominant character only, produces ripe germ-cells half of which bear 

 the dominant character of one parent, the other half, tlie recessive charac- 

 ter of the other parent. Thus, if the polydactylous Doi'king is crossed 

 with the normal Leghorn, nearly all of the hybrids will be polydac- 

 tylous — not quite all, however, for the extra toe in this case is not 

 complett'.ly dominant. But continued breeding shows that the sperm 

 and ova of the crossbreds will bear eitlier the dominant polydactylous 



