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 



