more, Dr.
Hrdlicka thinks, it might reach a point where one could speak of the
members of old American families as of a distinct stock. But so far this
point has not been reached; the Americans are almost as diverse and
variable, it appears, as were their first ancestors in this country.

APPENDIX D

THE ESSENCE OF MENDELISM

It is half a century since the Austrian monk, Gregor Mendel, published
in a provincial journal the results of his now famous breeding
experiments with garden peas. They lay unnoticed until 1900, when three
other breeders whose work had led them to similar conclusions, almost
simultaneously discovered the work of Mendel and gave it to the world.

Breeding along the lines marked out by Mendel at once became the most
popular method of attack, among those who were studying heredity. It
became an extremely complicated subject, which can not be grasped
without extended study, but its fundamentals can be briefly summarized.

Inherited differences in individuals, it will be admitted, are due to
differences in their germ-plasms. It is convenient to think of these
differences in germ-plasms (that is, differences in heredity) as being
due to the presence in the germ-plasm of certain hypothetical units,
which are usually referred to as factors. The factor, nowadays, is the
ultimate unit of Mendelian research. Each of these factors is considered
to be nearly or quite constant,--that is, it undergoes little, or no
change from generation to generation. It is ordinarily resistant to
"contamination" by other factors with which it may come in contact in
the cell. The first fundamental principle of Mendelism, then, is the
existence of relatively constant units, the Mendelian factors, as the
basis for transmission of all the traits that go to make up an animal or
plant.

Experimental breeding gives reason to believe that each factor has one
or more alternatives, which may take its place in the mechanism of
heredity, thereby changing the visible character of the individual plant
or animal in which it occurs. To put the matter a little differently,
one germ-cell differs from another in having alternatives present in
place of some of the factors of the latter. A given germ-cell can never
have more than one of the possible alternatives of each factor. These
alternatives of a factor are called its allelomorphs.

Now a mature germ-cell has a single system of these factors: but when
two germ-cells unite, the