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THE AMERICAN NATURALIST [Vol. L 



ods of modifying the chromosomes or carriers of the in- 

 herited qualities of organisms in order to further analyze 

 their normal physiological behavior; in the same way 

 that experimental embryology has been able to supply so 

 many valuable clues to the normal processes of devel- 

 opment. 



In the following pages we believe the facts indicate 

 that individual guinea pigs are now living in this experi- 

 ment that have had the carriers of hereditary qualities, 

 the chromatin, of their germ cells injured for a longer 

 time than four years. And during this time they have 

 given rise to offspring of more or less degenerate or de- 

 formed type, and in some cases these offspring have 

 passed this modified chromatin on through three genera- 

 tions, all of which contain pathological chromatin and 

 show somatic defects and deformities as an index of their 

 tainted chromatic ancestry. Modified chromatin has 

 been living in the experiment for more than four years 

 in five different generations of animals as a result of the 

 treatment on the one original, P 1? parent generation. 



We have tried to regulate every controllable source of 

 error and there can be no doubt that the conditions are 

 brought about in the way described. Could the degen- 

 eracy which is so pronounced have previously existed in 

 the stock? This question has been controlled in the first 

 place by the use of two entirely different stocks from dif- 

 ferent sources and obtained one and one half years apart, 

 the first in the fall of 1910 and the other in the early win- 

 ter of 1912. The responses of the two stocks to the ex- 

 perimental treatment have been identical. As a second 

 method of control every animal has been tested by one or 

 more normal matings before being introduced into the 

 experiment, and only those giving normally strong off- 

 spring have been used. A further crucial control is the 

 constant mating of normal untreated animals from both 

 stocks under identical cage conditions with the experi- 

 mental individuals. These animals continue to breed 



