58 



DE. E. E. GATES— CONTEIBUTION TO A 



metliods of analyzing those relationsliips. It is clear tliat forms wliich originate from 

 the same parent do not all have the same hereditary behaviour towards that parent. 

 Hence the relationship of their characters to the parent form is different in different 

 cases, and, therefore, the change implied in their method of origin has been different. 



The hereditary behaviour of the mutants of Oenothera is also usually different from that 

 of the related v.dld species. It seems justifiable to conclude that not only in Mendelian, 

 but in other types of hereditary behaviour, the method or type of inheritance is deter- 

 mined in large measure by the nature of the characters themselves. 



Our knowledge of heredity in Oenothera^ therefore, shows that several types of 

 inheritance exist, having an evident connection with the systematic relationships of the 

 forms, or, in other words, with the nature of the differentiating characters of those forms. 

 Blending and segregating inheritance both occur in Oenothera^ and it is not yet clear 



in how far the former may be explained in terms of the latter. 



VII. GenePwAl Considerations and Summahy. 



From the foregoing pages it will appear that the explanation of the mutation pheno- 

 mena in Oenothera Lamarckiana is by no means simple, and a full understanding of 

 the nature and cause of this behaviour has not yet been reached. A number of facts, 

 however, lead to the belief that the previous crossing which 0. LamarcMana has under- 

 gone, either in the wild condition or in Botanical Gardens, or both, has been the chief 

 cause of the germinal disturbances which make themselves manifest in the appearance 

 of mutants. The exact nature of these disturbances is not at present clear. Cytological 

 studies, as well as the hereditary behaviour, show that the various mutational changes 

 are of fundamentally different kinds, involving germinal alterations which are not only 

 diverse among themselves but even appear in some cases to originate at different points 

 in the life cycle. Thus, in O. gigas the essential change appears to have taken place in 

 the megaspore mother-cell or in the early divisions of the fertilized q^^, and possibly also 

 during the reduction divisions in the pollen mother-cell ; while the hereditary behaviour 

 of the original O. rtibricalyx mutant indicates that it was the result of a cross between 

 a mutated and a non- mutated germ-cell. On the other hand, it is not easy to explain the 

 behaviour of such mutants as O. ruhrinervis and O. nanella on the theory of their hybrid 

 or heterozygous origin, without involving a contradiction. O. ruhricalyx gives in each 

 generation a certain percentage of reversions to the parent form, O. ruhrinervis, until a 

 homozygous race is obtained, but in most of the other mutants no reversions occur. 



It seems entirely probable, as DeVries believes, that the cause of the mutations is 

 an internal one; and yet it cannot at present be decided whether this is due, for instance, 

 to irregularities in chromosome distributions or to the action of slight releasing stimuli. 

 In any case, the capacity for producing aberrant types is a result of germinal instability 



probably occasioned by previous crossing, the phenomena accompanying meiosis in 



the mutanis resembling those found in hybrids in various particulars. It is evident 

 that any explanation of mutation must take into account the peculiar chromosome 

 behaviour in this group. 



