EXTRA FINGERS AND TOES 
P | ‘HE occasional appearance of one 
or more fingers or toes in excess 
of the normal number is tech- 
nically known as polydactylism, 
and offers one of the most puzzling 
problems in heredity. Generally, the 
mode of inheritance of abnormalities 
of this sort is fairly plain and follows a 
simple scheme, but polydactylism has 
so far baffled all attempts to reduce it to 
rules; it appears to be almost anarchical. 
One of the reasons for this doubtless 
is that polydactylism may be due to a 
number of different causes. The 
guinea-pig may be cited in this connec- 
tion, for it offers abundant material 
for study. It ordinarily has three toes, 
but in 1905 Prof. W. E. Castle of Har- 
vard University found a four-toed speci- 
men which he bred and from which 
he has succeded in establishing a 
“four-toe” strain. It has now gone 
through fifteen or twenty generations, 
yet the character is not absolutely fixed. 
Pure-bred four-toes, when mated with 
their like, will occasionally produce 
three-toed individuals (which, however, 
are able to transmit the four-toe 
character to their offspring), just as 
pure-bred three-toed individuals when 
mated together will occasionally, al- 
though more rarely, produce a four-toed 
individual. Prof. Castle describes the 
heredity of the character as “partly 
blending, partly segregating;’’ that is, 
its inheritance is that of a simple 
Mendelian character, but one whose 
visible expression varies greatly. The 
character is neither dominant nor reces- 
sive, he holds, but is directly modified by 
crosses. “It is quantitatively variable,”’ 
he writes, ‘‘so that by selection one can 
establish high-grade or low-grade strains 
of polydactylism, and normals of poly- 
dactylous ancestry often transmit the 
character. It is as good a Mendelian 
character as many others, but (1) lacks 
dominance, (2) is variable, and (3) is 
affected by crosses; 7. e., blends to some 
extent or is contaminated. Strong sup- 
port for this interpretation is afforded 
by a recent paper on the inheritance of 
320 
flowering time in peas (Hoshino, 1915) 
in which it is shown that the character 
studied is subject to partial blending 
but is clearly Mendelian, for it is coupled 
with the color factor in crosses of red 
with white varieties.”’ 
MENDELISM IS QUESTIONABLE 
A Mendelian character with all the 
exceptional behavior which Dr. Castle 
describes is, however, a rather difficult 
character to follow, and many geneticists 
prefer to say that polydactylism in 
guinea-pigs is not yet demonstrated to 
be a Mendelian character. 
The explanation of the appearance of 
this fourth toe in guinea-pigs is simple: 
it represents merely a reversion to the 
ancestral condition. The ancestor of 
the guinea-pig, in fact, had five toes, 
and it is therefore likely that a fortunate 
geneticist will some day find this fifth 
toe cropping out, and thereby succeed in 
reestablishing a five-toed strain. Prof. 
J. A. Detlefsen of the University of 
Illinois actually found one such animal 
in his breeding experiments, but it was 
sterile and could not be used to produce 
a new race. 
Fowls, too, had five-toed ancestors, 
and although most birds of the present 
day have only four toes, a fifth toe 
sometimes appears. It might naturally 
be supposed that this represented the 
cropping out of the ancestral character; 
but Bateson and Davenport have shown 
that as far as the best known breeds of 
domestic fowl—the Dorkings and Hou- 
dans—are concerned, the fifth toe is 
not a reversion, but an abnormality 
due to the splitting of one of the toes 
(the hallux). How such a race might 
originate is graphically shown by the 
accompanying photograph (Fig. 16) of a 
Racing Homer pigeon, sent in by 
I. O’Neill Brenan of Brisbane, Queens- 
land. Here it is clearly seen that one 
of the toes has split, during develop- 
ment, producing a bird with five toes. 
This bird, bred to one of its own 
sisters, produced one squab that had an 
