18 The Journal of Heredity 
Inherited differences in individuals, 
it will be admitted, are due to differ- 
ences 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 hypo- 
thetical 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, if any, change from 
generation to generation. It is not 
“contaminated”’ by other factors with 
which it may come in contact in the 
cell. This is the idea which gives rise 
to the phrase “purity of the germ cells.”’ 
The first fundamental principle of 
Mendelism, then, is the existence of 
relatively constant units, the Mendelian 
unit-factors, as the basis for trans- 
mission of all the traits that go to make 
up an animal or plant. 
ALLELOMORPHISM 
From experimental breeding we find 
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, there result from that 
union two kinds of cells—namely, 
immature germ cells and body cells; 
and both these kinds of cells contain a 
double system of factors, because of 
course they have received a _ single 
entire system from each parent. This 
is the second of the fundamental 
principles of Mendelism: that the factors 
are single in the mature germ cell, but 
in duplicate in the body cell (and also 
in the immature germ cell). 
In every cell with a double system of 
factors, there are necessarily present 
two representatives from each set of 
allelomorphs, but these may or may 
not be alike—or in technical language 
the individual may be homozygous, or 
heterozygous, as regards the given set 
of alternative factors. Looking at it 
from another angle, we see a_ single 
visible character in the plant or animal, 
but it is produced by a double factor in 
the germ plasm. 
When the immature germ cell, with 
its double system of factors, matures, 
it throws out half the factors, retaining 
only a single system; and the allelo- 
morphic factors which then segregate 
into different cells are, as has been said 
above, uninfluenced by their stay 
together. 
But the allelomorphic factors are not 
the only ones which are segregated into 
different germ cells, at this maturation 
of the cell; for the factors which are not 
alternative. are likewise distributed, 
more or less independently of each 
other, so that it is largely a matter of 
chance whether factors which enter a 
cross in the same germ cell, segregate 
into the same germ cell or different ones, 
in the next generation. This is the 
next fundamental principle of Mendel- 
ism, usually comprehended under the 
term ‘‘segregation,”’ although, as has been 
pointed out, it is really a double process, 
the segregation of alternative factors 
being a different thing from the segrega- 
tion of non-alternative factors. 
From this fact of segregation, it 
follows that as many kinds of germ 
cells can be formed by an individual, 
as there are possible combinations of 
factors, on taking one alternative from 
each pair of allelomorphs present. In 
practice, this means that the possible 
number of different germ cells is almost 
infinitely great, as would perhaps be 
suspected by anyone who has tried to 
find two living things that are just alike. 
THE ESSENCE OF MENDELISM 
Such is the essence of Mendelism; 
and the reader is probably ready to 
admit that it is not a simple matter, 
even when reduced to the simplest 
terms. To sum up, the principal fea- 
