52 Split-Hand and Split-Foot Deformities 
homologous twins result from the fission of a single ovum. We can only conclude 
that in the case of Mayer's twins such an origin is highly improbable. 
Fig. 2. Fig. 3. 
Fig. 2. The figure illustrates the deformities of a family of hereditary split-foot described by Mayer 
(loc. cit.). The arrangement and signs correspond to those of Fig. 1. The family is of special 
interest as it shows: — marked dominance of the deformity: offspring from two wives by the 
original deformed member : male predominance : inheritance of type (II, 2 and 3, III, 10 and 11) : 
and homologous twins of which one is deformed (III, 6 and 7). 
Fig. 3. Diagram illustrating Mendelism as applied to human sports. The horizontal and oblique 
lines are to be interpreted as in Figs. 1 and 2. Each individual is represented by a double 
circle, the outer representing the appearance of the individual (black deformed and white normal), 
the inner circle divided by a vertical line the nature of the gametes. Homozygotes are consequently 
represented uniformly in black or white ; heterozygotes with the outer ring black (signifying 
the actual deformity and illustrating the dominance of the same) and the inner ring half black and 
half white, to denote the potential deformities of the gametic cells. In the figure a recessive 
and dominant homozygote are supposed to be crossed, producing heterozygotes only, one of which 
is shown. This individual is then crossed with a normal or recessive homozygote with the 
production of equal numbers of homozygotes and heterozygotes. Of these offspring one of each 
variety is crossed with a normal homozygote. The figure shows that from heterozygotes when 
crossed with normal individuals an equal number of deformed and normal offspring are to be 
expected. 
We now come to a subject of considerable interest and importance, in the 
discussion of the transmission of the deformity as a whole. There appears to have 
been of late years a growing feeling that slow or continuous variation is insufficient 
to account for the origin of widely differing species (cp. Lock), and a tendency to 
attribute such species to discontinuous variation or sport. The essential con- 
sideration in view of the possibility of a new species originating in a sport, is the 
stability of the particular deformity in its transmission. Weismann stated that 
there is no difficulty in understanding the gradual swamping of sports, when it 
occurs, and even went so far as to assert that it is inevitable in the absence of 
inbreeding. But Weismann based his statements chiefly on theoretical con- 
siderations, and the " id " hypothesis in its application to sport characters is 
perhaps no longer free from doubt. In the recent revival of Mendelism a great 
many new observations have been made into the subject of " hybridism." The 
investigations have been made for the most part upon the vegetable and lower 
