44 



THE AMERICAN NATURALIST [Vol.LVI 



change, its mutation must affect all of the cells in a hloc, 

 that are descended from the mutant cell. 



{c) The Conditions under which the Change occurs 

 But the method that appears to have most scope and 

 promise is the experimental one of investigating the con- 

 ditions under which mutations occur. This requires 

 studies of mutation frequency under various methods of 

 handling the organisms. As yet, extremely little has been 

 done along this line. That is because, in the past, a muta- 

 tion was considered a windfall, and the expression ' ' mu- 

 tation frequency ' ' would have seemed a contradiction in 

 terms. To attempt to study it would have seemed as 

 absurd as to study the conditions affecting the distribu- 

 tion of dollar bills on the sidewalk. You were simply 

 fortunate if you found one. Not even controls, giving the 

 normal " rate of mutation — if indeed there is such a 

 thing— were attcmptod." Of late, however, we may say 

 that certain very exce])tioiial banking houses have been 

 found, in front of wliich the dollars fall more frequently— 

 in otiicr words, s])(M'ially nnitable genes have been dis- 

 (■(»\cr(Ml. tli:it ;ii'c bcuiniiinii' to yield almndant data at 

 the l!;ui(l> of Xil>soii-l-:iil(', Zcleny, Emerson, Anderson 

 and others. Vov some of these mutable genes tlie rate of 

 cliange is found to be so rapid that at the end of a few 

 decades half of the genes descended from those originally 

 present would have become changed. After thes(^ genets 

 have once mutated, however, theii- prcx ious inntnhility no 

 longer holds. In addition to tlii- " li;nil<iiii:' hou-r> 

 method " there are also methods, eiiinl(i\ ( d hy A Itcnhuru: 

 aiHl nivM'lf, for as it were nnloni.'ilically sweeping up 

 wi(h. Ml-. ;,, nf the di-,-ct> Mtid dftiii- the collections for the 

 \ ;ilii;ihh'-. r,y \\]r-o >\)rA\\\ -viirtic iiK-thods of reaping 

 nmtatioii- wr h;i\r ivccinlv- >h(.\\ii that the ordinary 



