Long Life Means Many Children 
sure to live longer than the one who 
lacks an inherited resistance. 
But if each of them survived to adult 
life and left an equal number of children, 
it is obvious that the character of the 
race would not be changed—there 
would be relatively just the same 
numbers of weak and strong persons 
(in respect to tuberculosis-resistance) in 
it in the next generation, that there 
are in this. 
The theory of natural selection re- 
quires that the weaker of the two indi- 
viduals whom we have picked out for an 
illustration, must leave fewer offspring 
than the strong one. The less fit must 
leave fewer descendants than the more 
mt. hus the race progresses a little 
in each generation, in the direction of 
greater fitness in this respect. 
THEORY CONFORMS TO FACTS 
The theory requires, we have said, 
that there be this difference in fecundity. 
But do facts back up the theory? We 
have reached a point in the study of 
evolution where we are tired of theories. 
We want facts. 
101 
Sometimes we can get them; some- 
times we can not. In this particular 
case a study of long-lived people gives 
us exactly the information that we 
needed and expected. As we have seen, 
the long-lived—who must be considered 
to be in generally an unusually fit lot of 
people—are found to have more off- 
spring than the shorter-lived and less fit. 
And since this fitness is, as we know, 
hereditary, it follows that the race 
Should be getting a little more fit in 
this respect, with each generation, 
because the amount of inherited lon- 
gevity must be greater than it was in 
the previous generation. 
The lonesome and fatherly Mr. Davis, 
cited in a newspaper story at the begin- 
“ning of this article, may or may not 
exist in real life. But if he does not, it 
would not be difficult to find some one 
else conforming to the same specifica- 
tions; for long life does mean a large 
family. We have been able to measure 
the exact amount of association between 
the two facts, and thus add another 
piece of mathematically demonstrated 
proof to the theory of evolution which 
Darwin left us. 
Modes of Research in Genetics 
MopeEs oF RESEARCH IN GENETICS, by Ray- 
mond Pearl, Biologist of the Maine Agri- 
cultural Experiment Station. Pp. 182, 
price $1.25. New York, the Macmillan 
Company, 66 Fifth Avenue, 1915. 
The principal and most useful part 
of Dr. Pearl’s book is a discussion of the 
relative merits of the two _ principal 
methods used in studying heredity— 
namely, the biometric and the Men- 
delian. Public opinion is still confused 
as to the respective spheres of these 
methods, as a result of the conflict 
between them which raged a few years 
ago. Advanced students now generally 
recognize that each of these methods 
has its place, and also its limitations; 
but the beginner, who frequently reads 
the literature of a decade ago, is too 
often confused by the recriminations 
and denunciations he finds. Dr. Pearl 
shows clearly just what each method 
can do, the biometric method being 
essentially the study of the ancestry 
of an individual, while the Mendelian 
method is the study of the posterity of 
an individual. A perusal of this book 
will do much to clear up the ideas of a 
great majority of those who are ac- 
tively interested in genetics. The prob- 
lem of inbreeding, and the-general rela- 
tion between practical breeding and 
experimental genetics, are also discussed. 
Journal of Heredity As Text-book 
The class in heredity at the Ohio 
State University is now using the 
JOURNAL OF HEREDITY as its text-book, 
arrangements having been made for a 
special short-term membership in the 
American Genetic Association, for each 
of the twenty-two students taking the 
course. 
