74 The Journal 
are not easily accepted by the layman as 
furnishing conclusive evidence in the 
case of the higher animals. A number 
of breeding experiments with mammals 
are therefore being carried on in the 
United States at present, to throw more 
direct light on the problem of whether 
inbreeding is necessarily injurious. 
The Bureau of Animal Husbandry of 
the United States Department of Agri- 
culture has bred guinea-pigs for sixteen 
generations, on its farm at Beltsville, 
Md., mating brother and sister in each 
generation. No results of this work 
have yet been published. 
Darwin pointed out that swine seemed 
to be particularly susceptible to the evil 
effects of inbreeding; the Delaware Ag- 
ricultural Experiment Station is carry- 
ing on an experiment to test this point. 
Describing the work at the last meeting 
of the American Genetic Association, 
the director of the station, H. Hayward, 
declared that every line of descent had 
ended in disaster. Degeneration of the 
animals occurred, despite careful selec- 
tion, in each generation, to get the 
strongest animals for further breeding. 
It is now widely believed, by geneti- 
cists, that selection is the key to the re- 
sults of inbreeding. Obviously, when 
two parents are mated, each carrying a 
given character (because of their similar 
heredity), their offspring will get this 
character with increased intensity. If 
the character is good, it is argued, in- 
breeding ought to be beneficial in re- 
spect to this particular character. If 
the character is bad, however, the off- 
spring, getting a double dose of it, may 
naturally be expected to be inferior to 
its parents in regard to that character. 
Ali that is necessary, then, we are 
told, is to select for further reproduction 
the best animals in each generation. 
By so doing the evil effects generally 
attributed to inbreeding can be at 
least postponed indefinitely. 
Such a statement conforms to a 
widely-accepted theoretical view of in- 
breeding. It has not seemed to produce 
good results in the Delaware station’s 
pig breeding; but in another careful 
test it has vindicated itself completely. 
This test is the experimental breeding 
of Heredity 
of albino rats by Dr. Helen Dean‘King 
at the Wistar Institute, Philadelphia, 
Pa. It was first started to determine 
whether inbreeding alters the sex-ratio 
by increasing the relative number of 
males, as a number of investigators 
have maintained. Negative results 
being reached on that point, it was 
continued to test whether strains of 
rats with disproportionate sex-ratios 
could be produced by selection. The 
success on these lines was described in 
the January issue of the Journal of 
Heredity (Vol. VII, pp. 9-11). 
It is from the point of view of simple 
inbreeding, however, that the experi- 
ment is of interest here. Its scope will 
be understood from the fact that it is 
now in the twenty-second rat genera- 
tion, that a thousand rats are being 
produced in each generation at the 
present time, and that about 10,000 
inbred rats have, up to date, been 
produced, and a large number of them 
carefully weighed and described at 
regular intervals. 
The experiment was begun six years 
ago with a stock litter of four albino 
rats, two male and two female. From 
these two pairs have come all the rats 
since produced, and each line of descent 
has been kept separate, so that there are 
two series, A and B. In each of these 
series the closest inbreeding has been 
practiced in an unvaried way, brother 
and sister being in every case mated. 
About twenty selected females in each 
series are reserved, in each generation, 
to continue the experiment, and each of 
these is mated twice to a brother from 
the same litter. 
Although these females are carefully 
selected from the large number available, 
it must be noted that the basis of 
selection is primarily a disproportionate 
sex-ratio, and the general good qualities 
of the animal are taken into considera- 
tion only secondarily. But as far as is 
possible without conflicting with the 
primary interest of the experiment— 
namely, the question of sex determina- 
tion—the best rats are selected in each 
generation, size, vigor, fecundity and 
similar characters being taken into. 
account. In other words, Dr. King is. 
i ee we 
