EYE COLOR IN DROSOPHILA MELANOGASTER 383 



It seems probable that the bulk of the mutations that have 

 been permanent contributions to evolution have been those of 

 slight somatic change. Any organism as it now exists must be 

 regarded as a very complex physicochemical machine with deli- 

 cate adjustments of part to part. Any haphazard change made 

 in this mechanism would almost certainly result in a decrease 

 of efficiency. The greater the extent of the change the more 

 certain the injury, not simply that the particular part is injured 

 more, but also that a disproportionately greater number of ad- 

 justments (morphogenetic, physiological, and ecological) are 

 disturbed or destroyed. Only an extremely small proportion 

 of mutations may be expected to improve a part or the inter- 

 relation of parts in such a way that the fitness of the whole 

 organism for its available environments is increased. Thus, 

 that length of trunk which is most advantageous for elephants 

 in a given environment might be attained by a single mutation; 

 but in this case it is likely that the trunk would be out of balance 

 with the other structures (physiologically or otherwise) so that 

 all the individuals possessing this feature might become extinct 

 before the appearance of changes in these other parts that would 

 make the trunk a real success. On the other hand, a mutation 

 which made only a slight change in the favorable direction would 

 require less extensive supporting changes in the related structures, 

 besides which it might be of immediate advantage. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 28, NO. 3 



