^^o. 642] VARIATIOX l\ IXDIVIDCAL GENF 



pair of genes will detci niinc the existence of a particular 

 enzyme (concerned in pigment production), that another 

 pair of genes will determine whether or not a certain 

 agglutinin shall exist in the blood, a third pair will deter- 

 mine whether homogentisic acid is secreted into the urine 

 ("alkaptonuria"), and so forth. But it would be 

 absurd, in the third case, to conclude that on this account 

 the gene itself consists of homogentisic acid, or any- 

 related substance, and it would be similarly absurd, there- 

 fore, to regard cases of the former kind as giving any 

 evidence that the gene is an enzyme, or an agglutinin-like 

 body. The reactions whereby the genes produce their 

 ultimate effects are too complex for such inferences. 

 Each of these effects, which we call a " character" of 

 the organism, is the product of a highly complex, intri- 

 cate, and delicately balanced system of reactions, caused 

 by the interaction of countless genes, and every organic 

 structure and activity is therefore liable to l>ecome in- 

 creased, 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. 



IT. The Problem of Gene Mutability 

 The most <li>tiiictiv(' cliaracteristic of each of these 

 ultr:i-itiicn)sc()])ic partidi's — that characteristic whereby 

 we identify it as a gene — is its property of self-propaga- 

 tion: the fact that, within the complicated environment 

 of the cell protoplasm, it reacts in such a way as to 

 convert some of the common surrounding material into 

 an end-product identical in kind with the original gene 

 itself. This action fiiHills the cliemi^t's (h-Hiiition of 

 " autocatalysis "; it is wliat the pli> >i..h.,oist would call 

 " growth"; and when it ]ia>-es thi'ouuh more than one 

 generation it becomes h<'i'e(Iit \ . " ' It may lie observed 

 that this reaction is in ea.-h in^tniiee a rather liighly 

 localized one, since the new matei ial is hiid down by tlie 

 side of the original gene. 



