364 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
Primula kewensis. The original plant, which was sterile, ‘had 18 
and 9 chromosomes in its premeiotic and postmeiotic nuclei respec- 
tively,’ but in the fertile plants which were propagated asexually 
from it, as well as in similar fertile hybrids which were produced in 
later experiments, the diploid and haploid numbers were 36 and 18 
respectively. Having found by means of careful measurements of the 
chromosomes in the two forms that the nuclei in both forms contain 
the same volume of chromatin, the authors conclude that the increase 
in number may be attributed to transverse fission of the 18 larger 
chromosomes and not to the fusion of two nuclei. 
“From a study of chromosomal dimensions in relation to phylo- 
geny, Meek ‘arrived at the conclusion that the widths of chromosomes 
are successively greater in higher zodlogical phyla, and that this 
dimension is constant for very large groups of animals.’ But Farmer 
and Digby have shown that such a conclusion is-without foundation 
since ‘closely related forms may possess chromosomes differing widely 
in shape and size and character.’ Hence they conclude ‘that phylo- 
genetic affinity is not, necessarily, correlated with chromosome width.’ 
They also point out that ‘unfortunately we know practically nothing 
about the phylogeny of the chromosomes. No convincing hypothesis 
has been put forward to explain how these remarkable bodies have 
become organized, nor how their peculiarities have either been brought 
into existence or are kept so true for a given species.’ However, we 
are reminded by Glaser that chromatin is present in bacteria though 
not in the form of a nucleus and it may not be too much to hope that 
cytology may yet discover the principal stages in the development of 
the chromosomes and establish such correlation as may exist between 
this development and organic evolution. Certainly extended investi- 
gations of chromosome numbers must be made before chromosome 
aberrations can be considered an important factor in evolution. 
Except that certain chromosome aberrations, such a tetraploidy 
causing gigantism, might be of economic value, in general this class 
of mutations is of minor importance in breeding.” 
[Conclusion.—In bringing this discussion of the causes of heritable 
variations (mutations) to a close, we find ourselves in a somewhat 
pessimistic frame of mind. When all is said, it is found that our 
knowledge of what actually causes mutations is almost nothing. We 
think we know something about the mechanism of heredity, but we 
do not know the mechanism of variation. The really great evolution- 
ary discovery of the future will probably be the finding out of the 
cance ar the causes af mutations. —Ep.]1 
