2 



The mutable unit of heredity 



If all organisms were alike, the question of inheritance might never 

 have arisen. That "like produces like would be the natural order of 

 things. But were the question to arise in the ranging mind of some 

 philosopher, the science he would found would be a most difficult one. 

 It would involve the question: What is it about organisms that makes 

 them develop identically? This is, in fact, the problem of embryology, 

 and its answer is as important as it is elusive. But organisms do 

 differ, so that we can ask the question in another way: What is it about 

 one organism that makes it different from another? It is the differences 

 that proxide the observational units of genetics. It was the analvsis 

 of the inheritance of differences among garden peas that led Mendel to 

 the discovery of his laws. Similarly, when an unusual white-eyed fruit 

 fly was captured in a Columbia University laboratory, it was bred with 

 its red-eyed relatives as the first of a classic series of experiments 

 by Thomas Hunt Morgan and his students that led to the establishment 

 of the chromosomal theorv of heredity. 



Differences among organisms of the same species are not only indis- 

 pensable for the study of heredity but are also the stuff from which new 

 species are made. The mutations that initiate these differences are the 

 driving force in evolution. They provide the raw material for the inter- 

 action of the processes of heredity with selective forces in the environ- 

 ment; thus they permit the change that is so characteristic of life. It 

 has been supposed that mutations yield only harmful, inferior characters. 

 As a matter of fact, studies on organisms "higher" than bacteria show 

 that this is often the ca.se. But we must remember that organisms as we 

 find them have already been highly perfected during evolution. Mutant 

 genes of most kinds must have been tested as an assemblage for survival 

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