In the case of _Oenothera muricata x biennis_ the differentiating units
reduce the fertility to a low degree, threatening the offspring with
almost complete infertility and extinction. But then we do not know
whether these characters are really units, or perhaps only seemingly so
and are in reality composed of smaller entities which as yet we are not
able to segregate. And as long as we are devoid of empirical means of
deciding such questions, it seems useless to go farther into the details
of the question of the sterility. It should be stated here however, that
pure varietal crosses, when not accompanied by unbalanced characters,
have never showed any tendency to diminished fertility. Hence there can
be little doubt that the unpaired units are the cause of this decrease
in reproductive power.
The genus _Oenothera_ is to a large degree devoid of varietal
characteristics, especially in the subgenus _Onagra_, to which
_biennis_, _muricata_, _lamarckiana_ and some others belong. On the
other hand it seems to be rich in elementary species, but an adequate
study of [263] them has as yet not been made. Unfortunately many of the
better systematists are in the habit of throwing all these interesting
forms together, and of omitting their descriptive study. I have made a
large number of crosses between such undescribed types and as a rule got
constant hybrid races.
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