MULLER 



109 



progress made in the study of one of 

 them should help in the solution of 

 the other. 



Great opportunities are now open 

 for the study of the nature of the 

 synaptic attraction, especially through 

 the discovery of various races having 

 abnormal numbers of chromosomes. 

 Here we have already the finding by 

 Belling, that where three like chro- 

 mosomes are present, the close union 

 of any two tends to exclude their close 

 union with the third. This is very 

 suggestive, because the same thing is 

 found in the cases of specific attrac- 

 tions between inorganic particles, that 

 are due to their force field patterns. 

 And through Bridges' finding of tri- 

 ploid Drosophila, the attraction phe- 

 nomena can now be brought down to 

 a definitely genie basis, by the intro- 

 duction of specific genes— especially 

 those known to influence chromosome 

 behavior— into one of the chromo- 

 somes of a triad. The amount of influ- 

 ence of this gene on attraction may 

 then be tested quantitatively, by gen- 

 etic determination of the frequencies 

 of the various possible types of segre- 

 gation. By extending such studies to 

 include the effect of various condi- 

 tions of the environment— such as 

 temperature, electrostatic stresses, etc. 

 —in the presence of the different ge- 

 netic situations, a considerable field is 

 opened up. 



This suggested connection between 

 chromosome behavior and gene struc- 

 ture is as yet, however, only a pos- 

 sibility. It must not be forgotten that 

 at present we can not be sure that the 

 synaptic attraction is exerted by the 

 genes themselves rather than by local 

 products of them, and it is also prob- 

 lematical 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-important prob- 

 lem. 



It may also be recalled in this con- 

 nection that besides the genes in the 

 chromosomes there is at least one sim- 

 ilarly autocatalytic material in the 

 chloroplastids, which likewise may be- 

 come permanently changed, or else 

 lost, as has been shown by various 

 studies on chlorophyll inheritance. 

 Whether this plastid substance is sim- 

 ilar to the genes in the chromosomes 

 we can not say, but of course it can 

 not be seen to show synaptic attrac- 

 tion, and could not be studied by the 

 method suggested above.^ 



IV. THE ATTACK THROUGH STUDIES OF 

 MUTATION 



There is, however, another method 

 of attack, in a sense more direct, and 

 not open to the above criticisms. That 

 is the method of investigating the in- 

 dividual gene, and the structure that 

 permits it to change, through a study 

 of the changes themselves that occur 

 in it, as observed by the test of breed- 

 ing and development. It was through 

 the investigation of the chajiges in the 

 chromosomes— caused by crossing 

 over— that the structure of the chro- 

 mosomes was analyzed into their con- 

 stituent 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 re- 

 cent finding of changes in atoms and 

 investigation of the resulting pieces, 

 that has led us to the present analysis 

 of atomic structure into positive and 

 negative electrons having characteris- 

 tic arrangements. Similarly, to under- 



3 It may be that there are still other ele- 

 ments in the cell which have the nature of 

 genes, but as no critical evidence has ever 

 been adduced for their existence, it would 

 be highly hazardous to postulate them. 



