360 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
basis for this suspicion is that the type found in Holland is not a truly 
wild indigenous species, but a domestic type escaped from cultivation. 
-It seems probable that the species was imported from America a great 
many years ago. B. M. Davis has succeeded in producing by cross- 
ing two American wild species a hybrid form distinctly resembling 
Oenothera lamarckiana in numerous respects, and this hybrid, like any 
other hybrid, produces numerous combinations of the parental charac- 
ters when inbred, and these hybrid progeny sometimes resemble the 
mutants observed by De Vries. It is also said that the pollen grains 
of Oenothera lamarckiana exhibit a high degree of sterility, which is 
a characteristic defect of hybrid plants. 
Whether or not, however, the Oenothera situation be taken as 
valid evidence of the occurrence of mutations, the idea of mutations 
and their réle in evolution will stand up on quite independent grounds. 
Numerous mutations have been observed in all sorts of animals and 
plants. Professor T. H. Morgan and his able corps of collaborators 
have observed in their carefully controlled breeding experiments with 
the fruit fly Drosophila melanogaster hundreds of suddenly appearing 
new characters which are classed by them as mutants, and whose 
heredity has been most accurately studied. These mutants involve 
single characters of the organism which are sometimes of a prominent 
and readily recognizable sort and sometimes of so slight a degree as 
to be imperceptible except to the trained eye of the expert student of 
genetics. It also frequently occurs that two mutants are to the eye 
practically identical, but may be distinguished by differences in 
hereditary behavior. The great majority of the mutants discovered 
in Drosophila are termed “lethals” because they involve the death 
of the mutants possessing the ‘variation. A further discussion of 
Drosophila “mutants” appears in a later chapter.—Eb.] 
CAUSES OF MUTATIONS 
[Various mutations have been produced experimentally by subject- 
ing the germ cells to radically changed environmental conditions. 
W. L. Tower claims to have produced at least two new elementary 
species of potato beetle by subjecting the already grown and unchange- 
able parents to radically changed temperature and humidity conditions. 
Although the parents could undergo no change themselves they 
produced a small number of offspring with distinctly changed charac- 
ters. These turned out to be mutants because they bred true to the 
