Mutations under Glass Practical breeders and horticulturists bring 

 into the market every year beautiful new colorings among flowers and new 

 varieties of prize- winning animals. But most of these novelties do not con- 

 tinue long. They are replaced by other novelties. Sometimes this is a matter 

 of fashion and interest. At other times, however, the breeders are unable to 

 maintain a consistent variety for several generations. This has been the case 

 especially when novelties have arisen as the result of mating two different lines. 

 These hybrids are said to break up in succeeding generations, or to throw back 

 to the ancestral characteristics. The tremendous improvement in our under- 

 standing of heredity since the beginning of the century has made it possible 

 to follow closely plants and animals under controlled conditions. 



Among the most intensively studied animals were the famous fruit flies of 

 Professor Thomas H. Morgan (1866- ), of Columbia University and later 

 of the California Institute of Technology. The fruit flies are of no known 

 value in practical affairs. They were used only for convenience, for they can 

 be kept in large numbers in a comparatively small space. They have distinct 

 characteristics, which make it easy to study them with reference to particular 

 traits. And they reproduce at short intervals so that some twenty-five gen- 

 erations a year can be studied without too great cost or effort. 



Under these controlled conditions, Morgan and his associates were able to 

 observe in almost every generation from one to several mutations. Some of 

 the departures from the ancestral pattern reappeared in subsequent genera- 

 tions. In considering the rise and reproduction of these various fruit flies, no 

 question is raised as to the adaptive value of the new qualities. In many cases, 

 indeed, the freak was unable to reach maturity or to reproduce itself. Nor for 

 the moment was any question raised as to what feature in the general environ- 

 ment, in the food, or in the strain itself brought about such mutations. It was 

 necessary merely to make sure that the freak arose in a "pure line" — that is, 

 was not itself the result of crossing, or "hybridizing" — and that the new 

 characters reappeared in the offspring. 



Similar observations have been made with many kinds of plants, as well 

 as with other animals, in all parts of the world. Literally thousands of muta- 

 tions have been described, and they have furnished a valuable basis for the 

 interpretation of the problems of inheritance. 



Mutations in the Making Speculation as to the cause of a mutation led 

 to experiments with the various factors of the environment. The effects of 

 temperature, chemical conditions, dryness, changes in the food, have all been 

 tried. In 1928 H. J. Muller (1901- ), then of the University of Texas 

 and since working in research laboratories in different parts of the world, 

 showed that under certain conditions X rays produced marked effects upon the 

 germ substance of mature fruit flies. Treating cultures of insects with X rays 

 increased the proportion of mutations in the following generation. This 



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