The Making of Species 
FERTILE CROSSES BETWEEN SPECIES OF PLANTS 
In the case of plants the number of fertile 
hybrids between species is so large that we 
cannot attempt to enumerate them. De Vries 
cites several instances in Lecture IX of his 
Species and Varieties: Their Origin by Mutation. 
One of these—the hybrid between the purple 
and the yellow species of Lucerne which is 
known to botanists as Medicago media is, writes 
De Vries, “cultivated in some parts of Germany 
on a large scale, as it is more productive than 
the ordinary lucerne.” Other examples of per- 
fectly fertile plant hybrids cited by De Vries 
are the crosses between Anemone magellanica 
and A. sylvestris, between Salix alba and Sahx 
pentandra, between Rhododendron hirsutum and 
Rk. ferrugineum. 
He gives an instance of a hybrid—Zgzlops 
spelteformts, which, though fertile, is not so 
fertile as a normal species would be. It is worthy 
of note that Burbank of California has obtained 
a hybrid between the blackberry and the rasp- 
berry, which is not only fertile, but quite popular 
as producing a novel fruit. 
STERILE Ptanr Hysrips 
De Vries does not cite nearly so many examples 
of sterile hybrids, presumably because they are 
not so easy to find. He mentions the sterile 
118 
