Xo. 042] VABIATION IN INDIVIDUAL GENE 



not be sure that the synaptic attraction is exerted by the 

 genes themselves rather than by local products of them, 

 and it is also problematical whether the chief part of the 

 mechanism of autocatalysis resides within the genes 

 rather than in the protoplasm." Meanwhile, the 

 method is worth following up, simply because it is one 

 of our few conceivable modes of approach to an all-im- 

 portant problem. 



It may also be recalled in this connection that besides 

 the genes in the chromosomes there is at least one sim- 

 ilarly autocatalytic material in the chloroplastids, which 

 likewise may become permanently changed, or else lost, 

 as has been shown by various studies on chlorophyll inher- 

 itance. Whether this plastid substance is similar to the 

 genes in the chromosomes we can not say, but of course 

 it can not be seen to show synaptic attraction, and could 

 not be studied by the method suggested above.* 



IV. TiiK Attack tii hough Stvdies of ;N[vtatiox 

 TJiere is, however, anotlior method of attack, in a sense 

 more direct, and not open to the above criticisms. That 

 is the method of investigating the individual gene, and 

 the structure that i>ermits it to cliange, through a study 

 of the changes themselves that occur in it, as observed 

 by the test of breeding and development. It was through 

 the investigation of the changes in the chromosomes- 

 caused by crossing over — that the structure of the chro- 

 mosomes was analyzed into their constituent genes in 

 line formation ; it was through study of molecular changes 

 that molecules were analyzed into atoms tied together in 

 definite ways, and it has been finally the rather recent 

 finding of changes in atoms ;ni<l iiixcstigation of the 

 resulting pieces, that has IimI hs to flie ]n'(^sent analysis 

 of atomic structure into posit i\-.' and n(\uativt> electrons 

 having characteristic arrangements. Similarly, to under- 

 stand the properties and possibilities of the individual 

 gene, we must study the mutations as directly as possible, 

 and bring the results to bear upon our problem. 



