II. THE MUTATED GENE AND THE POTENTIALITIES 

 OF DEVELOPMENT 



In the following discussions, we take the existence of the gene 

 for granted without further inquiry into its nature, as the facts 

 that we shall have to report and the conclusions to be drawn arc 

 largely independent of the conception- regarding the nature 

 of the gene. One of the basic facts of genetics is, then, thai the 

 action of the gene in controlling the typical development of 

 hereditary traits cannot be studied directly but may only be 

 extrapolated from the knowledge of the action of a mutant gene: 

 the existence of the normal or +- gene is only inferred from the 

 existence of a mutant allelomorph showing Mendelian behavior. 

 The action of a mutant gene is a different one from the assumed 

 action of the -f— gene; i.e., the mutant gene must control or 

 produce a deviation in the series of developmental processes 

 leading to the visible character. Development, as we know, is, 

 leaving aside the difficult problem of regulation, a process of 

 extraordinary precision. Under constant conditions, one step 

 follows the other with almost invariable precision in regard to 

 space and time. Each step is dependent upon the normal appear- 

 ance of the preceding one, and the normal result depends — 

 barring regulations upon the orderly sequence of events in 

 quality, quantity, time, and space. One of the consequences of 

 this situation is that the possibilities of changing the details of 

 developmental processes without injuring the proper cooperation 

 of those processes are rather limited. Most of the changes of 

 individual processes that might be produced will throw out of 

 gear the combined system, and the result will be destructive. 

 But certain processes may be changed without deleterious 

 consequences; and if tin- i> done by a genetic change, we call it 

 a mutation. Such considerations, obvious as they seem to be, 

 make US expect that the action of mutated scenes upon develop- 

 ment cannot he of a different type from any other changes of 



development induced by experimental agencies: in both cases, 

 something changes the detailed course of some developmental 



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