PRENTISS: POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 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 Kquidae), 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? The objection is raised, however, 
that there is too great a distance in point of time and relationship between 
the polydactyle animal and the pentadactyle ancestor to which it ¢s 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, the recessive charac- 
ter of the other parent. Thus, if the polydactylous Dorking 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 
completely dominant. But continued breeding shows that the sperm 
and ova of the crossbreds will bear either the dominant polydactylous 
