ON THE NATURE OF MUTATIONS 



Their Cause to be Sought in a Chemical Rather Than Morphological Change in 



Some Chromosome — New Material Furnished for Evolution — 



Data from the Evening Primroses.' 



Dr. R. Ruggles Gates, University of London. 



EVER since the beginning of the 

 twentieth century, when the 

 principles of Mendel were redis- 

 covered and DeVries published 

 his mutation theory, the nature of the 

 changes which give rise to mutations 

 and to Mendelian differences has been 

 much discussed. For it is obvious that 

 our views on these questions will 

 determine our attitude toward evolution 

 in so far as it is concerned with discon- 

 tinuity in variation. Mendelian in- 

 vestigators have for the most part 

 assumed that the main point to be 

 discovered is the manner of inheritance 

 of the Mendelian differences found in 

 plants and animals, and have failed to 

 observe that the origin of these differ- 

 ences is a wholly different question, 

 requiring in many cases other methods 

 of attack. 



It has also been tacitly or specula- 

 tively assrmied that mutations were in 

 some obscure and hypothetical way 

 the expression merely of new combina- 

 tions of Mendelian units. This view, 

 or rather group of views (for each 

 Mendelian writer has attempted a 

 different hypothesis, such as reduplica- 

 tion of gametes, coupling of characters, 

 multiple factors, loss of inhibitors, etc.), 

 has only recently been exploded. But 

 it is worth while pointing out that even 

 if this were not the case it would still 

 be necessary to account for the origin 

 of the supposed Mendelian differences. 

 This Mendelian writers have attempted 

 to do by means of the presence-absence 

 hypothesis. But it will be shown later 

 that the latter hypothesis has had 

 placed upon it a far greater weight of 

 speculation than it is able to bear. 



The activities of recent years in the 

 study of mutations have, on the con- 



trary, accumulated a mass of positive 

 observations concerning the real nature 

 of mutational changes. These investi- 

 gations have combined cell studies with 

 observations of the external characters 

 of mutants and their hereditary beha- 

 vior, both in self-fertilized offspring and 

 in crosses with their parent forms. In 

 this way much light has been thrown 

 upon the nature of mutation as a process 

 of variability. It has been definitely 

 shown that mutation is a type of varia- 

 tion and cannot be ex]3laincd in terms 

 merely of heredity, Mendelian or other- 

 wise. The cell studies of mutants in 

 particular have been crucial in elimi- 

 nating the various Mendelian hypoth- 

 eses of mutation, for the latter are found 

 to be directly contrary to the cytological 

 facts. These facts have also made it 

 possible to construct a new theory of 

 mutations in general which helps to 

 illiuninate the whole subject and har- 

 monize the Mendelian and mutationist 

 views. 



We are thus led to adopt a new 

 attitude toward the subject of heredity as 

 well as that of variation ; the latter being 

 concerned with the origin of differences 

 between related organisms, while the 

 former is concerned with the perpetua- 

 tion of those differences after they have 

 originated. 



EVIDENCE FROM THE PRIMROSE. 



The critical and decisive experiments 

 concerning the nature of mutations have, 

 as is well known, been made in the eve- 

 ning primrose genus Oenothera. Here 

 the numerous attempts to explain the 

 origin of the mutants in terms of the 

 recombination of Mendelian factors 

 have broken down. For a nvmiber of 

 years it has been assiuned in various 



1 Presented before the Botanical Society of America, Philadelphia, Dec. 30, 1914. 



99 



