CAN WE SEE EVOLUTION OCCURRING? 



as well as from both of the parents/ These differences show 

 themselves in the characteristics of the developed individuals, 

 in thousands of diverse ways, some very marked, some 

 extremely inconspicuous. It becomes therefore extremely 

 difficult to distinguish differences produced in this way — 

 by recombinations of genes in biparental reproduction — from 

 differences that are steps in evolution. In most higher 

 organisms this is indeed at the present time impossible. 



Yet in certain higher organisms these difficulties have 

 been overcome. By study, continued for years, the hereditary 

 constitutions of the parents are thoroughly learned, so that 

 the results of their combination are known. In such organ- 

 isms the hereditary constitution does change at times, irre- 

 spective of the recombinations due to the union of two 

 parents. The changes are infrequent. Yet in such an animal 

 as the fruit-fly {Drosophila), studied by Morgan and his 

 disciples, so great has been the number of individuals and of 

 generations minutely studied that literally hundreds of dif- 

 ferent alterations in hereditary characters have been observed. 

 Drosophila has given rise to hundreds of new stocks, which 

 differ permanently from the original one. Some of the 

 changes are strongly marked, as when red-eyed animals sud- 

 denly produce a white-eyed race, or when long-winged crea- 

 tures suddenly produce a race that is permanently without 

 wings. These very marked changes were naturally the first 

 ones observed, so that for a long time it was believed that 

 all evolutionary changes were large leaps, saltations. But 

 since acquaintance with the animals has become more minute 

 it has been discovered that extremely slight, almost imper- 

 ceptible, changes in hereditary characters are much more 



^ For details as to this process of recombination of genes in reproduction 

 from two parents, see any modern text-book of genetics ; e.g., T. H. Morgan's 



The Physical Basis of Heredity. 



[31] 



