738 THE AMERICAN NATURALIST. [Vol. XXXVII. 



gations have, or should have, their chief purpose in the discovery 

 of the physical mechanism of heredity. 



The terms discontimioiis variation, or mutation in connection 

 with the study of inheritance, descent, and the origin of species 

 may be taken to mean the autonomous physiological processes 

 by which one or more individuals of a species give rise to off- 

 spring which exhibit qualities, or groupings of qualities not pos- 

 sessed by their immediate ancestors and not previously exhibited 

 by the individuals comprised in the parent species (progressive 

 mutation), or by which one or more individuals give rise to indi- 

 viduals lacking qualities or groupings of qualities exhibited by 

 the ancestral forms (retrogressive and degressive mutation). 

 These aberrant individuals or mutants may transmit their char- 

 acters to their offspring in such a manner as to give rise to a 

 new line of descent constituting the origin of a new type by 

 mutation. 



The number of freaks, sports, bud-variations, and specimens 

 of plants with abnormal forms and sizes of leaves, stems, and 

 flowers, some of them highly teratological, to which attention 

 has been called by various writers in botanical periodicals under 

 the designation of mutants makes necessary the emphasis of 

 the fact that observations on a single individual, or a single gen- 

 eration of individuals are of but little value in distinguishing 

 fluctuating variations from mutations. Results worth a moment's 

 consideration may be obtained only by the most careful exclusion 

 of the possible effects of disease, of animal or plant parasites, 

 of hybridization, and by a careful analysis of the phylogenetic 

 value of the divergences as tested by observations on successive 

 generations of living forms. It is in this manner, and in Ibis 

 manner alone, that discontinuous, saltatory variations may be 

 distinguished from the results of common, fluctuating and indi- 

 vidual variability. Mutation rests in the main upon such sub- 

 stantive, discontinuous variations as the acquisition of new 

 characters, or the loss of old ones hitherto transmitted by the 

 parent type, or upon simultaneous alterations of both kinds. 

 These changes may be accompanied by, or may result in, the 

 masking of current qualities, or the unmasking and energizing 

 of latent quahties of the parent type. 



