6 



DE. E. E. GATES— C0:NTRIBUTI ON TO A 



occasional irregularities in the distribution of chromosomes durino- meiosis, which 



omo 



furnish a possible basis for the origin of negative mutants, provided that the 

 somes in Oenothera are qualitatively different. Eut since the chromosomes of these 

 plants are morphologically undifferentiated, there is at present no opportunity of 

 following this possibility further. Chromosome distributions in the germ-cells are 

 doubtless concerned in the origin of mutants having 15 or 16 cliromosomes. 



To return to the difficulty regarding the hypothesis that a mutation is of hybrid 

 nature, arising from the fusion of a mutated with an unaltered cell, let us suppose, for 

 instance, that O. ruhrinervis arises from a mutated germ-cell crossed with an ordinary 

 0. LamarcUana germ-cell. If this is so, why does O. ruhrinerms breed true when self- 

 pollinated, yet when crossed with O. LamarcUana it splits in the F^ giving O. LamarcUana 

 and O. ruhrinervis, both of which remain constant in later generations ? It might be 

 said that the primary "mutated" germ-cell was different from a ruhrinervis germ-cell, 

 but that when crossed with O. LamarcUana it gave a combination which bred true and 

 produced the ruhrinervis characters. However, if this were admitted, there is no reason 

 for supposing that our ruhrinerms germ-cell when crossed with O. LamarcUana would 

 behave in an entirely different manner and give alternative inheritance. To explain the 

 origin of a mutation on this basis, one must assume a change in the hereditary 

 behaviour of the germinal material in two successive generations where one would 

 expect it to be the same. Why should a mutated germ-cell, crossed with a non- 

 mutated one, give a blend which never splits, while the germinal material derived from 

 this blend, when crossed with another non-mutated germ-cell of O. LamarcUana, always 

 exhibits alternative inheritance ? Such a condition of affairs is conceivable but not 

 probable. It involves a paradox which has not yet been explained away. 



Leclerc du Sablon (1910) has formulated the ingenious hypothesis that the phenomena 

 of mutation in O. LamarcUana are to be explained as a case of Mendelian splitting in 

 which there is a very high degree of " coupUng " between the characters. But°this 

 seems to be merely putting the facts into another notation without really adding to our 

 knowledge of wliy the new forms appear. Moreover, it fails to take account of the 

 change in chromosome number in O. gigas. This nuclear change places O. gigas in a 

 different category from the other mutants. 



I have already mentioned the hypothesis that the mutants may be due to occasional 

 irregularities in the distribution of the meiotic chromosomes. Another, and possibly 

 more probable, explanation is that though the mutations are internal in origin, yet they are 

 released by an external stimulus which affects the cytoplasm, and indirectly the nuclear 

 structure. If this is an approach to the true explanation, the releasing stimuli must be very 

 slight. This view is difficult to harmonize with the fact that the mutants seem to appear 

 with ahout the same frequency wherever O. LamarcUana is cultivated. However, 

 comparatively little is known concerning such environmental responses in organisms. 



i'inally, we may agree that mutation, in so far as it is distinguished from hybrid- 

 splitting (see p. 47), probably accounts for much species-formation. Many polymorphic 

 genera are probably indebted to such a process for the large number of closely 



