of the children show the lock and half
do not. Photograph from Newton Miller.]

[Illustration: A FAMILY OF SPOTTED NEGROES

FIG. 20.--The piebald factor sometimes shows itself as nothing
more than a blaze in the hair (see preceding figure); but it may take a
much more extreme form, as illustrated by the above photograph from Q.
I. Simpson and W. E. Castle. Mrs. S. A., a spotted mutant, founded a
family which now comprises, in several generations, 17 spotted and 16
normal offspring. The white spotting factor behaves as a Mendelian
dominant, and the expectation would be equal numbers of normal and
affected children. Similar white factors are known in other animals. It
is worth noting that all the well attested Mendelian characters in man
are abnormalities, no normal character having yet been proved to be
inherited in this manner.]

Apart from multiple factors as properly defined (that is, factors which
produce the same result, either alone or together), extensive analysis
usually reveals that apparently simple characters are in reality
complex. The purple aleurone color of maize seeds is attributed by R. A.
Emerson to five distinct factors, while E. Baur found four factors
responsible for the red color of snapdragon blossoms. There are, as G.
N. Collins says,[49] "still many gross characters that stand as
simple Mendelian units, but few, if any, of these occur in plants or
animals that have been subjected to extensive investigation. There is
now such a large number of characters which at first behaved as units,
but which have since been broken up by crossing with suitable selected
material, that it seems not unreasonable to believe that the remaining
cases await only the discovery of the right strains with which to
hybridize them to bring about corresponding results."

In spite of the fact that there is a real segregation between factors as
has been shown, it must not be supposed that factors and their
determiners are absolutely invariable. This has been too frequently
assumed without adequate evidence by many geneticists. It is probable
that just as the multiplicity and interrelation and minuteness of many
factors have been the principal discoveries of genetics in recent years
that the next few years will see a great deal of evidence following the
important lead of Castle and Jennings, as to variation in factors.

Knowing that all the characters of an individual are due to the
interaction of numerous factors, one m