Review: Mendelism Up to Date eatao 
view held by a large number of those 
who have gone most deeply into the 
subject. : 
“Exception,’’ he continues, “may 
perhaps be taken to the emphasis we 
have laid on the chromosomes as the 
material basis of inheritance. Whether 
we are right here, the future—probably 
a very near future—will decide. But 
it should not pass unnoticed that even 
if the chromosome theory be denied, 
there is no result dealt with in the fol- 
lowing pages that may not be treated 
independently of the chromosomes; for, 
we have made no assumption concern- 
ing heredity that cannot also be made 
abstractly without the chromosomes as 
bearers of the postulated hereditary 
factors. Why, then, we are often asked, 
do you drag in the chromosomes? Our 
answer is that since the chromosomes 
furnish exactly the kind of mechanism 
that the Mendelian laws call for, and 
since there is an ever-increasing body of 
information that points clearly to the 
chromosomes as the bearers of the 
Mendelian factors, it would be folly 
to close one’s eyes to so patent a rela- 
tion. Moreover, as biologists, we are 
interested in heredity not primarily as 
a mathematical formulation but rather 
as a problem concerning the cell, the 
egg, and the sperm.”’ . 
To this the reviewer can add that 
the chromosome view appears to have 
gained ground in America during the 
past year or two, among these who are 
most competent to hold an opinion on 
the subject; and that despite the strong 
opposition of some biologists of note, no 
alternative explanation has been put 
’ ‘ 
forward which has met with any except 
a limited acceptation. 
Finally, to sum up the main features 
of the mechanism of Mendelian hered- 
ity, as understood by the authors of the 
book under discussion, and many other 
geneticists, we find among our ideas the 
following: 
1. That the various characters which 
make up the physical constitution of any 
individual plant or animal are due to the 
action (concurrently with the environ- 
ment, of course) of what we term, for 
convenience, factors, separable hypo- 
thetical units in the germ-plasm, capable 
of independent transmission. 
2. That each visible character is due 
to the cooperative action of an indefi- 
nitely large number of factors (for such 
a simple creature as the fly Drosophila, 
there may be ten or twenty millions); 
conversely, that each of these factors 
affects an indefinitely large number of 
characters. 
3. That these factors, or their ma- 
terial bases, are passed from one genera- 
tion to another in certain bodies called 
chromosomes, in the egg and sperm. 
4. That the factors are generally 
linked together in groups, each chro- 
mosome having a group of its own; that 
they are arranged along the chromo- 
some in a linear series, but sometimes 
change places with each other by 
“crossing over.’’ To these propositions 
there are a number of corollaries which ° 
cannot here be mentioned. 
If the conclusions above listed stand 
the test of time in anything like their 
present form, genetics can well challenge 
every other science to produce a greater 
body of results in fifteen years. 
High Fecundity in Hens Not a Unit Character 
An editorial footnote attached to Slocum’s article on poultry breeding in the 
November issue of the JOURNAL OF HEREDITY (p. 485) unintentionally misrepre- 
sented the work of Dr. Raymond Pearl and the Maine Experiment Station, by stating 
their conclusions to be based on the belief that high egg production in hens is a 
unit character. 
This statement was entirely incorrect. 
Dr. Pearl has presented 
data in several papers to show that fecundity in poultry is a character influenced 
by at least three separate and distinct heritable factors. 
that he thought it a unit character. 
He has never stated 
