Review: Mendelism Up to Date 19 
tures at the base of the hypothetical 
structure are these: 
1. There exist relatively 
units in the germ plasm. 
2. There are two very distinct rela- 
tionships which these units may show 
to each other. Two (or more) unit 
factors may be alternatives in the 
mechanism of inheritance, indicating 
that one is a variation (or loss) of the 
other; or they may be independent of 
each other in the mechanism of inher- 
itance. 
3. The mature germ cell contains a 
single system of independent factors 
(one representative from each set of 
alternates). 
The immature germ cells, and body 
cells, have double systems of inde- 
pendent factors (two from each set of 
alternatives). 
4. The double system arises simply 
from the union of two single systems 
(2. e., two germ cells), without union or 
even contamination of the factors 
involved. 
In the formation of a single system 
(mature germ cells) from a double 
(immature germ cells), pairs of alter- 
nates separate, passing into different 
germ cells. Factors not alternates may 
or may not separate—the distribution 
is largely a matter of chance. This 
chance distribution is more or less 
disturbed by the phenomenon of linkage, 
which will be described later in this 
review. 
constant 
INFLUENCE OF WEISMANN 
Such are the fundamental principles 
of Mendelism; but on them was early 
grafted a theoretical structure due 
mainly to the German zoologist, August 
Weismann. To understand his part in 
the story, we must advert to that much- 
mooted and too often misunderstood 
problem furnished by the chromosomes. 
These little rods of easily stained ma- 
terial, which are found in every cell of 
the body, were picked out by Weismann 
as the probable carriers of heredity. 
With remarkable acuteness, he pre- 
dicted their behavior at cell-division, 
the intricate nature of which is usually 
the despair of every beginner in biology. 
When Mendelian breeding, in the early 
years of this century, showed temporary 
pairing and subsequent separation of 
units in the germ cell, it was soon 
realized that the observed facts of 
breeding fitted to a nicety the observed 
facts (predicted by Weismann) of chro- 
mosome-behavior; for at each cell- 
division the chromosomes, too, pair 
and separate again. The observed be- 
havior of transmitted characters in 
animals and plants followed, in so 
many cases, the observed behavior of 
the chromosomes, that many students 
found it almost impossible to believe 
that there was no connection between 
the two, and Weismann’s prediction, 
that the chromosomes are the carriers 
of heredity, came to be looked on as a 
fact, by many biologists. 
But when this much of Weismann’s 
system was accepted, other parts of it 
went along, including a hypothetical sys- 
tem of ‘‘determiners”’ in the chromo- 
some, which were believed to determine 
the development of characters in the 
organism. Every trait of an animal or 
plant, it was supposed, must be repre- 
sented in the germ-plasm by its own 
determiner; one trait, one determiner. 
Did we find a notch in the ear running 
through a pedigree? Then it must be 
due to a determiner for a notch in the 
ear in the germ-plasm. Did we find 
mathematical ability hereditary? Then 
there must be a determiner, the expres- 
sion of which was mathematical ability. 
For a while, this hypothesis was of 
service in the development of genetics; 
some students even began to forget that 
it was a hypothesis, and to talk as if it 
were a fact. But the exhaustive tests 
of experimental breeding of plants and 
animals have long caused most of the 
advanced students of genetics to drop 
this simple hypothesis. 
In its place, we have the factorial 
hypothesis, evolved by workers in Amer- 
ica, England, and France at about the 
same time, and of which the work of 
Morgan and his associates at Columbia 
University is one of the solid bases. It 
is the hypothesis accepted by a majority 
of the leading American geneticists at 
the present day; unfortunately it 1s 
scarcely apprehended by many who are 
not actively working in genetics, and 
