144 
must find facts of other kinds and 
throw them into the scales. It hap- 
pens that the facts set forth in this 
book fall into the side of the scale 
marked environment. By and by we 
shall have more facts. As we dig them 
out we must carefully inspect them to 
see whether they belong in one scale 
or the other. It is easy to mistake the 
scale in which a given fact should fall, 
and sometimes we may have done so in 
this book. Yet even so there remain 
many facts which indicate that ex- 
tremes of heat and cold, moisture and 
dryness, are somehow associated with 
pronounced changes in the form and 
function of organs of the body. This 
single fact, if it be a fact, is more 
important than all else that we have 
The Journal of Heredity 
here discussed. Part of its importance 
lies in that it opens up the possibility 
that some day mankind may learn not 
only how to select the best variations 
in a given plant or animal, but how to 
cause a great number of widely diverse 
mutations from which he may select. 
“Tn all this the human race is merely 
one among the species of animals. For 
aught we know, his migrations and the 
many new and artificial conditions to 
which he subjects himself may be alter- 
ing some of his most deep-seated quali- 
ties. We spend millions in the attempt 
to improve plants and animals. Is it 
not time that we learned how the high- 
est of all the animals is being changed 
and how his future evolution may be 
directed along the right path?” 
Morgan on 
Tue PuysicaL Basis oF Herepity, by 
Thomas Hunt Morgan, professor of 
experimental zoology in Columbia 
University. Monographs of Experi- 
mental Biology; the J. B. Lippin- 
cott Co., Philadelphia, 1919. Pp. 305, 
with 117 illus. Price, $2.50. 
During the past twenty years the na- 
ture of the process of inheritance has 
been demonstrated in detail to the satis- 
faction of nearly every one, and no man 
has had a larger part in this grea 
accomplishment than Dr. Morgan, The 
present book is the most complete ac- 
count extant of the mechanism of 
heredity, and it will therefore be indis- 
pensable to every serious student of the 
subject, even though in some respects 
it will not at once supplant “The Mech- 
anism of Mendelian Heredity,’ which 
Dr. Morgan and his associates published 
in 1915. 
All of the important or moot points 
of the subject are discussed, and diffi- 
culties are met squarely, except in a 
few instances, as in a discussion (p. 36) 
Heredity 
of the objection that Mendelism deals 
only with superficial characters, such as 
color, This is on its face a fundamen- 
tal objection, and the only answer Dr. 
Morgan makes is to cite the well-known 
lethal factors that destroy the individual 
when homozygous. “There can be no 
question as to: the fundamental impor- 
tance of such factors,’ he truthfully 
states; but certainly this does not an- 
swer the attack, and it might as well be 
admitted that the characters whose in- 
heritance has so far been worked out 
satisfactorily are in general superficial 
characters. It is easy enough to see 
that any important structure or func- 
tion must be due to the interaction of a 
large number of factors, and it is no 
cause for apology that geneticists have 
not yet been able to isolate all the fac- 
tors that go to make such a character. 
The book contains a good _ bibli- 
ography, which brings a fresh realiza- 
tion of the great amount of work that 
has been done in genetics in the brief 
time that it has existed as a science.— 
pee: 
