126 W. J. MOENKHAUS 
’ 
INBREEDING AND SELECTION ON FERTILITY AND VIGOR 
1. Introductory 
That continued inbreeding acts deleteriously on the fertility 
and vitality of a race is a belief so firmly and generally established 
that it is seldom questioned. This has its origin largely in the 
common experience of breeders whose observations, unfortun- 
ately, are too often unreliable. There are not wanting experi- 
ments such as those of Van Guaita (’98) and Bos (’94) and others, 
scientifically conducted, which bear out this conclusion. 
On the other hand, it is refreshing to encounter in the literature 
such reports as that of Gentry (’05) who believes not only that 
inbreeding is not necessarily harmful, but also that it may be 
beneficial to conserve and intensify the good points in his breed. 
Gentry’s experiments were made on Berkshires. The most pro- 
longed tests of close inbreeding that have been recorded were 
made by Castle (’06) on the same species with which the present 
paper deals. He inbred (brothers with sisters) for fifty-nine gener- 
ations. He concludes that such close inbreeding does not neces- 
sarily result in a loss of productiveness and of vigor; at least that 
inbreeding cannot be regarded as a causal factor. Some of his 
results so nearly parallel those of the present writer that further 
reference to his results will be made in the body of the paper. 
During the early part of October, 1903, a number of pairs were 
started breeding. These came from various sources in Bloomington. 
These different pairs were reared for the most part only a few 
generations, excepting pair No. 6 which was continued for about 
four and one-half years. During this time over seventy-five 
generations were produced. Toward the close of this period no 
exact count was kept of the generations so that only an approxi- 
mate figure can be given. Five pairs of brothers and sisters were 
mated in each generation to insure against accidents that might 
terminate the strain if but one mating were made. 
Along at the fifth and sixth generation it became more and 
more difficult to keep the strain alive with the five pairs of brothers 
and sisters that were mated each generation. The failure of an 
