Experimental Inbreeding 75 
trying to avoid evil effects from inbreed- 
ing in exactly the way that geneticists 
have told the practical breeder to go; 
she is picking out only the best breeders 
in each generation, and _ discarding 
all the unfit.” 
RESULTS ARE GOOD 
The result seems to have demon- 
strated that the geneticists were right 
when they advised such a course—at 
least, as far as rats are concerned. For 
these laboratory rats, which have been 
inbred as closely as possible for twenty- 
two generations, are in every respect 
superior to the stock rats from which 
they took their start six years ago, and 
which have since then been bred in the 
usual indiscriminate manner. 
Beginning with the seventh generation 
of the experiment, from three to five 
litters in each generation of each series 
have been weighed, first at 13 days of 
age, then at intervals of one month 
until they were 15 months old. Their 
growth has been carefully compared 
with that of the stock rats, bred under 
the same environmental conditions. 
Growth in body weight is practically 
the same in inbred and in stock albino 
rats during the first sixty days of 
postnatal life, both in the males and 
in the females. After this time, how- 
ever, inbred rats exceed the stock rats 
in body weight at any givenage. After 
150 days of age, inbred males are about 
15% heavier than stock males. In the 
case of the females, which are always 
smaller than males, there is much less 
difference between the growth of the 
stock and inbred rats; nevertheless 
inbred females are, throughout adult life, 
about 3% heavier than stock females. 
When it is remembered that selection 
for size has been only secondary, 
selection for sex-ratio being the first 
consideration, this increase in body 
weight appears to be a very striking 
accompaniment of the inbreeding which 
has been done. 
What of the sterility which is said, 
sometimes, to be an inevitable result of 
inbreeding? The rats certainly do not 
show any falling off in that respect. 
“We have found that the average litter 
of stock albino rats contains seven 
young,’’ Dr. King says.’ ‘Records for 
over 1,200 litters of inbred rats show 
that the average number of young per 
litter is 7.4. Litter size in the rat, 
therefore, is seemingly increased and not 
diminished by inbreeding, even when, 
as in these experiments, there has been 
no direct attempt to increase the fer- 
tility. 
“Under the conditions in our colony 
inbred rats live fully as long as do stock 
rats and they appear equally resistant to 
disease; so their constitutional vigor 
does not seem to be impaired as yet.”’ 
The reader will perhaps wonder 
whether there are not a great many 
“wasters” produced in each generation— 
animals defective in vigor or in some 
other respect. It can be said that the 
proportion of defectives appears to be 
no larger in the inbred series than in the 
stock rats. Furthermore, no female se- 
lected for breeding has ever proved 
sterile. 
Dr. King sums up the experiment to 
date as follows: 
DR. KING'S CONCLUSION 
“The results so far obtained with 
these rats indicate that close inbreeding 
does not necessarily lead to a loss of size 
or of constitutional vigor or of fertility, 
if the animals so mated come from sound 
stock in the beginning and sufficient care 
is taken to breed only from the best in- 
dividuals.” 
In the face of the data she presents, 
one can hardly refuse to accept this con- 
clusion. 
It must be noted, however, that the 
experiment can not, in the nature of 
things, settle the problem of whether 
inbreeding is, of itself, injurious. The 
experiment proves conclusively that if 
2 The only abnormalities so far noted in the inbred strain, since external conditions were uni- 
form, are: one tailless rat, killed by mother soon after birth; half a dozen cases of absence of one 
or both eyeballs. 
This latter defect is equally common in the rats that are not inbred. 
3 In an address before the Pediatric Societies at Philadelphia on November 9, 1915. 
From 
this address (which has not been published) most of the facts herewith presented are derived. 
