THE MUTATION THEORY 495 



phology of the nucleus. Such alterations in the conformation of the 

 nuclear material are to be regarded as germinal changes, even though 

 they are not accompanied by external changes in the organism. 



MUTATION" 

 H. J. MULLER 



Beneath the imposing building called "Heredity" there has been a 

 dingy basement called "Mutation." Lately the searchlight of genetic 

 analysis has thrown a flood of illumination into many of the dark 

 recesses there, revealing some of them as ordinary rooms in no wise 

 different from those upstairs, that merely need to have their blinds 

 flung back, while others are seen to be subterranean passageways of 

 quite a different type. In other words, the term mutation originally 

 included a number of distinct phenomena, which, from a genetic point 

 of view, have nothing in common with one another. They were classed 

 together merely because they all involved the s.udden appearance of a 

 new genetic type. Some have been found to be special cases of 

 Mendelian recombination, some to be due to abnormalities in the 

 distribution of entire chromosomes, and others to consist in changes in 

 the individual genes or hereditary units. It seems incumbent upon us, 

 however, in the interests of scientific clarity, to agree to confine our use 

 of the term mutation to one coherent class of events. The usage most 

 serviceable for our modern purpose would be to limit the meaning of 

 the term to the cases of the third type — that is, to real changes in the 

 gene. This would also be most in conformity with the spirit of the 

 original usage, for even in the earlier days, mutations were conceived 

 of as fundamental changes in the hereditary constitution, and there 

 were never intentionally included among them cases merely involving 

 redistribution of hereditary units — when these cases were recogniz- 

 able as such. In accordance with these considerations, our new defini- 

 tion would be: "mutation is alteration of the gene." And "alteration," 

 as here used, is of course understood to mean a change of a transmissible, 

 or at least of a propagable, sort. 



In thus trimming down the scope of our category of mutation we 

 do not deprive it of the material of most fundamental evolutionary 

 significance. For all changes due to the redistribution of individual 

 genes or of groups of genes, into new combinations, proportions, or 

 quantities, are obviously made possible only by the prior changes that 



' Reprinted from Eugenics, Genetics and the Family, Vol. I (1923). Courtesy of 

 the Williams and Wilkins Company. 



