106 



countless genes, and every organic 

 structure and activity is therefore 

 liable to become increased, diminished, 

 abolished, or altered in some other 

 way, when the balance of the reaction 

 system is disturbed by an alteration in 

 the nature or the relative quantities of 

 any of the component genes of the 

 system. To return now to these genes 

 themselves. 



II. THE PROBLEM OF 

 GENE MUTABILITY 



The most distinctive characteristic 

 of each of these ultra-microscopic par- 

 ticles—that characteristic whereby we 

 identify it as a gene— is its property 

 of self-propagation: the fact that, 

 within the complicated environment 

 of the cell protoplasm, it reacts in such 

 a way as to convert some of the com- 

 mon surrounding material into an end- 

 product identical in kind with the 

 original gene itself. This action fulfills 

 the chemist's definition of "autocatal- 

 ysis"; it is what the physiologist would 

 call "growth"; and when it passes 

 through more than one generation it 

 becomes "heredity." It may be ob- 

 served that this reaction is in each 

 instance a rather highly localized one, 

 since the new material is laid down by 

 the side of the original gene. 



The fact that the genes have this 

 autocatalytic power is in itself suffi- 

 ciently striking, for they are un- 

 doubtedly complex substances, and it 

 is difficult to understand by what 

 strange coincidence of chemistry a 

 gene can happen to have just that very 

 special series of physico-chemical 

 effects upon its surroundings which 

 produces— of all possible end-products 

 —just this particular one, which is 

 identical with its own complex struc- 

 ture. But the most remarkable feature 

 of the situation is not this oft-noted 

 autocatalytic action in itself— it is the 

 fact that, when the structure of the 



MULLER 



gene becomes changed, through some 

 "chance variation," the catalytic prop- 

 erty of the gene may ^ become corre- 

 spondingly changed, in such a way as 

 to leave it still autocatalytic. In other 

 words, the change in gene structure- 

 accidental though it was— has some- 

 how resulted in a change of exactly 

 appropriate nature in the catalytic re- 

 actions, so that the new reactions are 

 now accurately adapted to produce 

 more material just like that in the new 

 changed gene itself. It is this paradoxi- 

 cal phenomenon which is implied in 

 the expression "variation due to 

 change in the individual gene," or, as 

 it is often called, "mutation." 



What sort of structure must the 

 gene possess to permit it to mutate 

 in this way? Since, through change 

 after change in the gene, this same 

 phenomenon persists, it is evident that 

 it must depend upon some general fea- 

 ture of gene construction— common to 

 all genes— which gives each one a gen- 

 eral autocatalytic power— a "carte 

 blanche"— to build material of what- 

 ever specific sort it itself happens to 

 be composed of. This general principle 

 of gene structure might, on the one 

 hand, mean nothing more than the 

 possession b\^ each gene of some very 

 simple character, such as a particular 

 radicle or "side-chain"— alike in them 

 all— which enables each gene to enter 

 into combination with certain highly 

 organized materials in the outer proto- 

 plasm, in such a way as to result in 

 the formation, "by" the protoplasm, 

 of more material like this gene which 

 is in combination with it. In that case 

 the gene itself would only initiate and 

 guide the direction of the reaction. On 

 the other hand, the extreme alternative 

 to such a conception has been gen- 



1 It is of course conceivable, and even 

 unavoidable, that so7?ie tj'pes of changes do 

 destroy the gene's autocatalytic power, and 

 thus result in its eventual loss. 



