264 GENETICS IN RELATION TO AGRICULTURE 
that doubling of the chromosome number may occur during somato- 
genesis has been found by Farmer and Digby in the interesting hybrid, 
Primula kewensis. The original plant, which was sterile, ‘‘had 18 and 9 
chromosomes in its -premeiotic and postmeiotic nuclei respectively,” 
but in the fertile plants which were propagated asexually from it, as 
well as in similar fertile hybrids which were produced in later experi- 
ments, 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 phylogeny, 
Meek ‘‘arrived at the conclusion that the widths of chromosomes are 
successively greater in higher zoological 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 phylogenetic 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 chromo- 
somes and establish such correlation as may exist between this develop- 
ment and organic evolution. Certainly extended investigations of 
chromosome numbers must be made before chromosome aberrations 
can be considered an important factor in evolution. Except that 
certain chromosome aberrations, such as tetraploidy causing gigantism, 
might be of economic value, in general this class of mutations is of minor 
importance in breeding. 
Factor mutations, on the other hand, are of prime importance and of 
general occurrence. Factor mutations have appeared in controlled 
cultures of many animals and plants and the character differences con- 
ditioned by them are as a rule such as distinguish varieties of a single 
species. Moreover, varietal characters are Mendelizing characters 
in the narrow sense and the existence of simple Mendelian phenomena 
among all classes of sexually propagated organisms proves that factor 
mutations are of general occurrence. Although it is probable that every 
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