Other Kinds of Hybridizing 159 
De Vries has found that some of the elementary species of the 
evening primrose also show a certain degree of infertility when 
crossed ; and there can be little doubt that infertility may begin 
with the appearance of elementary species, and increase in pro- 
portion to every new change in the germ that takes place. 
Whether infertility is a general rule for elementary species, may 
be questioned. 
Many cases of crossing between wild species of animals and 
plants have been recorded as occurring in nature, and many 
more cases have been experimentally brought about, especially 
in plants, with wild forms kept under domestication. The re- 
sults are different in different cases, but it is a generally accepted 
opinion that the species-cross is generally intermediate be-_ 
tween the parents. This conclusion needs, perhaps, careful 
revision in the light of the results of recent years on crossing types 
that differ in only one character, where in many cases discontinu- 
ous inheritance is the rule. Darwin was so impressed with the 
difference in the results of crossing Linnean species and sports 
that he concluded that wild species could not have arisen as 
sports, since the latter when crossed show discontinuous inheri- 
tance, while wild species give intermediate forms. Since Dar- 
win’s time our knowledge of the results of hybridizing has greatly 
increased, and his argument seems less conclusive, because, in 
the first place, a single mutation may show incipient infertility, 
as in de Vries’s cenotheras; in the second place, because the 
results of crossing elementary varieties and elementary species 
with the parent forms or with each other do not always show 
discontinuous inheritance; thirdly, because wild species have 
undergone so many changes of different kinds that the results 
are too complicated for an analysis of single characters; and, 
fourthly, because discontinuous inheritance may sometimes oc- 
cur between wild species, if unit characters rather than the 
ensemble of characters is considered. It is the failure to recog- 
nize this last point that has probably led to an exaggerated idea 
of the difference between the inheritance of single variations and 
of complex variations that characterize Linnean species. 
