22 The Journal of Heredity 
The curious and definite facts which 
have led to this conception must be 
dealt with in explaining heredity by any 
hypothesis, but seem to fall in exactly 
with the chromosome hypothesis which 
is adopted by the authors. We start 
with the idea that the factors are car- 
ried in the chromosomes, and that from 
time to time they cross over from one 
to another, after the manner diagramat- 
ically shown in Fig. 5. When the rela- 
tive frequency of the occurrence of the 
various possible crossovers was tabulated 
it was found that they were made with 
a certain regularity, which was hardly 
compatible with the idea that the factors 
were swimming around at large in the 
chromosome, or that they changed places 
without any provocation. It was found 
almost necessary to assume that they 
had definite stations, that each factor 
was regularly located at a certain defi- 
nite point in the chromosome. 
PROOF OF LINEAR ARRANGEMENT 
That assumption is exactly what the 
authors have made; and, as they have 
had a great number of cases of crossing 
over to study, they have been able 
actually to draw a map of the four 
chromosomes possessed by Drosophila, 
and to locate in them the various traits 
which they have studied. The correct- 
ness of their map can be tested in a very 
ingenious way: 
“The chance that such a process of 
crossing over will occur between any 
two given points on the chromosome 
should obviously be greater, the greater 
the distance between these points. If 
then the Mendelian factors lie along the 
chromosome, the amount of crossing 
over between any two of them will 
depend on their distance apart. Should 
two points lie near together a crossover 
will only rarely occur between them; 
if they lie further apart the chance of 
such a crossover taking place at some 
point between them will be greater. 
From this point of view the percentage 
of crossing over is the expression of the 
‘distance’ of the factors from each other.”’ 
In other words, this is a case where 
we can ask, and receive, definite mathe- 
matical evidence to support the idea 
of linear arrangement. We need only 
examine the behavior of a group of 
factors that are not too far apart. If 
we find that factors A and B are 
crossing over four times in a hundred 
opportunities, while factors B and C are 
giving twelve crossovers in the same 
number of chances, then we should 
expect to find that the number of crosses 
of A and C is either the sum or the 
difference of these two numbers—. e., 
sixteen or eight—depending on whether 
C hes to the one side or the other of 
A and B. Having ascertained the 
relative position of C, we then go on to 
D, E, and all the other factors that we 
find in the group, and which are fairly 
close together—for if they are far apart, 
the crossing over will be disturbed, for 
reasons that need not be described 
here. The authors say that they have 
actually got the definite numerical 
results expected, and these have enabled 
them to draw the chromosome map 
which they print as a frontispiece. 
The idea of linear arrangement of the 
factors, it should be added, depends at 
present almost wholly on the work of 
the authors, few other geneticists hav- 
ing suitable material for testing it. The 
authors have shown that it fits some of 
the results obtained by Bateson and 
his associates, which were explained by 
those experimenters on a different basis. 
It is not possible here to discuss any - 
of the other conceptions of Mendelian 
heredity, which the authors describe, but 
enough has perhaps been said to make 
the reader realize that the modern ideas 
of Mendelian heredity, differing widely 
from the earlier ideas which went under 
the same name, are decidedly complex, 
but exact. 
Just how far are they facts, and how 
far theories, the reader may well ask. 
It can only be replied that beyond the 
observed results of hybridization, all 
is hypothesis. Many who are engaged 
in the study of heredity do not even 
accept-the hypothesis that the chromo- 
somes are the carriers of the substances 
or factors which lie at the base of heredity. 
Nevertheless, Dr. Morgan is unques- 
tionably correct when he remarks in 
the preface that the view set forth in 
the book, a few features of which have 
been glanced at in this review, is the 
