CHAPTER XX 

 MUTATION 



Concerning the origin of the germinal differences that 

 give rise to mutant characters very little is known at pres- 

 ent except, (1) that they appear infrequently, (2) that the 

 change is definite from the beginning, (3) that some of 

 the changes at least are recurrent, and (4) that the differ- 

 ence between the old character and the new one is small 

 in some cases and greater in others. I do not think that 

 any of the work purporting to produce specific mutational 

 changes has succeeded in establishing its claims, at least 

 in the sense that we can pretend at present to control the 

 appearance of specific mutant changes, and until this is 

 done we can not hope to find out very much as to the 

 nature of these changes. Our study of the germ-plasm 

 is largely confined, therefore, for the present, to a study 

 of transmission of the genes, to the kinds of effects they 

 produce on the organism, and to the special relations of 

 the genes in the chromosomes where they are located. 



Concerning the frequency of mutation there is a slowly 

 increasing body of evidence showing in some animals 

 and plants how often or how rarely changes of this kind 

 take place. The impression prevails that mutation is less 

 rare in some species than in others, and while I am inclined 

 to think that this may be true, not much value can be 

 ascribed to such impressions ; for it is not improbable that 

 the frequency with which mutations are found is often 

 directly in proportion to the number of individuals exam- 

 ined and to familiarity with the type in question, so 

 that the smaller changes are not overlooked. The dis- 

 covery of new mutant types in almost every plant and 

 animal that has been carefully examined indicates at least 

 the very general occurrence of definite mutations, and the 



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