MUTATIO'NS AND EVOLUTION. 



By J. E. Duerden:. M.Sc, Ph.D., 



Professor of Zoology, Rhodes University College, Grahams- 

 town ; Officer-in-Charge, Ostrich Investigations, Groot- 

 fontein School of Agriculture, Middelburg, C.P. 



Read July 7, 1919. 



The variations with which the mutationist has hitherto been 

 mainly concerned are those of an isolated, discontinuous kind, 

 and it is generally assumed that they arise from factorial 

 changes, the result of mitotic irregularities during gametogenesis. 

 Thus, in his Presidential Address before the British Association 

 in Australia in 1914, Prof. W. Bateson* remarks : " The isolated 

 events to which variation is due are evidently changes in the 

 germinal tissues, probably in the manner in which they divide. 

 It is likely that the occurrence of these variations is wholly 

 irregular, and as to their causation, we are absolutelv wathout 

 surmise or even plausible speculation." 



Probably most species show numbers of disconnected muta- 

 tions which have arisen in this way. For the most part they 

 represent small iiidividtial differences, germinal in their nature, 

 and fluctuating alx)ut a mean. Prof. H. S. Jenningsf states: 

 " All thorough work has led directly to this result, that any species 

 or kind or organism is made up of a very great number of diverse 

 stocks, differing from each other in minute particulars, but the 

 diversities inherited from generation to generation." Prof. M. 

 Caulleryij: also remarks: "A species is nothing but the sum of 

 an infinity of genotypes differing very little from one another." 

 If the germ plasm of the species is in a fixed and stable condition, 

 then isolation of the pure germinal forms, genotypes, showing 

 the separate differences, becomes possible, as in Johannsen's well- 

 known experiments with beans and those of Jennings with 

 Paramecium ; but if it be in a changeable state, as in -Jennings' 

 work on Difflngia corona, genetic analysis into fixed pure races 

 is naturally impossible. How far the germ plasm of a form is 

 stable or unstable for the time being is a matter of observation 

 and experiment. 



These small variations are the germinal fluctuations of a 

 species, and under ordinar}^ circumstances the species will be 

 limited to a mixed j^pulation oi this kind. Most of the dif- 

 ferences will be indift'erent as regards their influence on the 

 welfare of the individual, and no change will take place in the 

 average assemblage ; if, however, among them one or more 



* " Science." Vol. XL, Aug. 28, 1914, p. .301 



X CI 



t 



Journ. Wash. Acad. Sc," vol. VII,' May iQ, 1917, p. 282. 



i "Science," vol. XLIII. April 21. 1916, p. 55^. 



