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THE AMERICAN NATURALIST [Vol. XLYIII 



assume only that constitutional changes usually occur in 

 the germ cells, but on this belief the sporting plants must 

 often be F 1 hybrids, and the plant breeder must resort to 

 selection to isolate his pure mutation. And by the same 

 reasoning one gametic change may produce many new- 

 creations, for there is a chance to recombine it with all the 

 known gametic differences in the species. 



No one can say how often mutations arise. It is likely 

 that changes other than the one observed took place in 

 our tobacco experiments, but it is not likely that they 

 are sufficiently numerous to base a system of selection 

 within a pure race on the possibility of their occurrence. 

 The fact that no changes ensued that could be detected in 

 several of our selected lines is an argument against it. 

 The comparatively large jumps are the ones likely to 

 have the greatest economic importance, and these are 

 easily detected without refined methods of procedure. 

 Small jumps can be economically important only if they 

 are numerous, and, as there are absolutely no data to 

 show either that they are numerous or that changes can 

 be produced rapidly within homozygous pure lines through 

 any other cause, it seems unwise to recommend that the 

 practical breeder expend time and money to bring about 

 results that either can not be expected at all or that are 

 so slow and so trifling that they can not be detected in 

 carefully planned and accurately executed genetic inves- 

 tigations. On the other hand, the results of the last de- 

 cade show that important economic results can be ob- 

 tained easily and surely by selection from artificial hy- 

 brids or from the natural hybrids that occur in cross- 

 fertilized species by the recombination of Mendelian 

 factors. We believe, therefore, that the isolation of ho- 

 mozygous strains from mixtures that are either mechan- 

 ical or physiological, that are either made artificially or 

 are found in nature, offers the only method of procedure 

 that the practical plant breeder will find financially 

 profitable. 



Finally, we should like to call attention again to the 



