EXPERIMENTAL INBREEDING 
Important Problem Tested With Various Animals and Plants—Rats Inbred by 
Dr. King for 22 Generations, Accompanied by Careful Selection and 
Followed by Excellent Results—The Lesson of Experi- 
mental Breeding for the Commercial 
Livestock Breeder. 
RACTICAL livestock breeders 
are, on the whole, firmly con- 
vinced that close inbreeding, that 
is, the mating of closely related individ- 
uals, is injurious, leading to degencra- 
tion and sterility. Their testimony was 
reviewed by Darwin! in a well-known 
chapter of his book on animals and 
plants under domestication; and he him- 
self made the first important experi- 
mental contribution to the subject, by 
his plant pollinations, which led him to 
express the opinion that cross-fertiliza- 
tion is generally beneficial. 
Ritzema-Bos (1894) was one of the 
first biologists to bring forward experi- 
mental evidence of value from animal 
breeding. During six years he contin- 
uously inbred a family of rats. In the 
first four years, covering 20 generations, 
no bad effects were observed, but in the 
following 10 generations the vigor and 
fertility of the race considerably de- 
clined. ‘‘The average-sized litter in the 
first half of the experiment was about 
7.5, but in the last vear of the experiment 
it had fallen to 3.2, and many pairs were 
found to be completely sterile. © Dimi- 
nution in size also attended the inbreed- 
ing, at the end amounting in the case of 
the males to between 8 and 20%.”’ 
At about the same time, Weismann 
carried on a similar experiment with 
mice, starting with nine individuals— 
six females and three males. ‘‘In the 
first ten generations the average number 
of young to a litter was 6.1; in the next 
ten generations it was 5.6; and in the 
last nine generations it had fallen to 4.2.”’ 
Such experiments gave support to the 
stock-breeder’s belief that inbreeding 
is, in the long run, injurious—although 
it is hardly necessary to point out that 
no practical breeder ever inbred to any- 
thing like the same extreme degree as 
was done in these laboratory experi- 
ments. Of the various cases of ex- 
treme inbreeding of large animals may 
be mentioned that of Lord Derby, who 
mated brother and sister for nine gener- 
ations of race horses, with distinctly bad 
results, and of J. Cossar Ewart, who in- 
bred the descendants of a pair of goats 
as closely as possible for nine genera- 
tions, until the strain practically “ran 
out.” 
Experiments with smaller animals, on 
the other hand, have shown that under 
some conditions close inbreeding may be 
carried on for a long time without bad 
results. Thus, in the case of the little 
fruit-fly, Drosophila, Castle and his 
pupils bred brother with sister for fifty- 
nine generations ‘‘without obtaining a 
diminution in either the vigor or the 
fecundity of the race, which could with 
certainty be attributed to that cause. 
A slight diminution was observed in 
some cases, but this was wholly obvi- 
ated when parents were chosen from the 
more vigorous broods in each genera- 
tion.”’ 
WOODRUFF’S LONG STUDY 
But the most extensive test of the kind 
is that of Woodruff, who carried the one- 
celled Paramecium, which usually repro- 
duces by simple division, through more 
than 3,000 generations, without any 
marked deterioriation in the stock. 
Such investigations, however, with 
fruit-flies, maize, and the like material, 
' The evidence cited by Darwin was reviewed at some length by H. Kraemer in the JOURNAL 
oF Herepity for May, 1914, Vol. V, No. 5, p. 226. 
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