3^4 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



Primula kewensis. The original plant, which was sterile, 'had i8 

 and 9 chromosomes in its premciotic 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 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 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 

 cause or the causes of mutations. — Ed.] 



