effects of recombination of the different sorts of qualifies
contained in the parental strains. His attack on the
hypothesis of rejuvenation that was so generally held
at that time did very great service in exposing the
mystical nature of such an imagined effect of cross-
fertilization. In particular, Weismann's endeavor to
connect the theory of recombination with the facts
of maturation of the egg and sperm has opened our
eyes to possibilities that had never been realized before.
His work has led directly to the third advance that
has been made in very recent years, when the results
of Mendelian segregation have been applied directly
to the study of fertility and sterility.

As I have said, Darwin's work showed that cross-
fertilization is generally beneficial. The converse
proposition has long been held that continued inbreed-
ing leads to degeneration and to sterility. This opinion
rests largely on the statements of breeders of domesti-
cated animals and plants, but there is also a small
amount of accurate data that seems to support this
view. I propose first to examine this question, and
then consider what cross-fertilization is supposed to do,
in the light of the most recent work.

Weismann inbred white mice for 29 generations,
and Ritzema-Bos bred rats for 30 generations. In
each case the number of young per litter decreased
in successive generations, more individuals were sterile
and many individuals became weakened. This evi-
dence falls in line with the general opinion of breeders.

On the other hand, we have Castle's evidence on
inbreeding the fruit fly through 59 generations. He
found some evidence of the occurrence of sterile pairs

-195-