THE EXPERIMENTAL STUDY OF GENETIC RELATIONSHIPS I43 
Phascum was anomalous in that it was dwarf, rather than giant. It 
was sterile, but reproduced itself by propagula, which do not usually 
occur in the parent form, Phascum cuspidatum. Many mosses re- 
produce almost exclusively by vegetative means, and this experiment 
is in the highest degree suggestive of the manner in which this habit 
has come about. All of the other bivalent races differed from their 
parents in characters which obviously depended upon the greater size 
of the individual cells. 
The experiments of the Marchals give us the strongest reason to 
believe that the visible differences between Oenothera Lamarckiana and 
Oe. gigas have been correctly interpreted as due to the doubling in th^ 
latter of the chromosome number. This explanation, first proposed 
by Gates, has been assented to by de Vries. The latter points out, 
however, that the genetical and physiological qualities of Oe. gigas 
are entirely different from those of Oe. Lamarckiana. The seeds of 
Oe. gigas, for example, are much more viable than those of the parent 
species, and germinate more quickly. Oe. gigas does not give the laeta 
and velutina twin hybrids when crossed with unrelated species, as 
Oe. Lamarckiana does. Obviously, then, the process of mutation has 
changed the hereditary qualities of the germ plasm even more than the 
morphological characters of the sporophyte. Oe. gigas gives rise to 
certain secondary derivatives which have no counterparts among the 
variations of Oe. Lamarckiana. Are we too optimistic if we view the 
former as a newly evolved center about which an entirely new series of 
specific variations may spring up? 
It is natural to ask at this point how commonly mutative changes 
in the chromosome number occur. It is known that in Oe. lata, one 
of the most characteristic of the mutations from Oe. Lamarckiana, the 
unreduced number is 15 instead of 14. In semigigas mutations from 
Oe. Lamarckiana and Oe. biennis the number is 21. I have recently 
investigated the mutability of several species of Oenothera which 
belong to the small-flowered, self-pollinating series of forms which are 
generally lumped together under the name "Oe. biennis.'^ Two of 
them have given rise to mutations characterized by an increased 
chromosome number. The cytological investigation of these new 
mutations has been undertaken by Mr. E. G. Arzberger, through whose 
kindness I am able to announce that Oe. stenomeres mut. gigas has 28 
chromosomes, and that Oe. pratincola mut. gigas also has this num- 
ber. The former seems in every way analogous to Oe. gigas de Vries, 
