12 BULLETIN 905, TJ. S. DEPARTMENT OF AGRICULTURE. 



ing cells undergo specialization into skin, muscle, bone, nerve cells, 

 etc., and never give rise to reproductive cells. The two classes of 

 cells, reproductive cells and body cells, thus have separate histories, 

 and any influence of one group on the other must be indirect. 



It will easily be seen that this leads to a very different conception 

 of heredity from that mentioned above. The reproductive cells are 

 not produced by the body. They are simply an unchanged bit of the 

 same material which previously developed into the body of the 

 parent. Heredity consists merely in their retention of the power 

 to develop into a complete individual under the proper conditions. 

 Thus, so far as heredity is concerned, the way in which an individual 

 is related to his parents and more remote ancestors does not differ 

 essentially from the kind of relationship with brothers, uncles, etc. 



This view of heredity was first reached by Sir Francis Galton, in 

 England, and Augus't Weismann, in Germany, from a consideration 

 of the history of the reproductive cells. Numerous experiments 

 have also been made to test its truth. A striking illustration is 

 given by an experiment performed by Prof. W. E. Castle and Dr. 

 John C. Phillips, of Harvard University. They removed the ovaries 

 of a female albino guinea pig and placed in her body the ovaries of 

 an immature black female, aged about 3 weeks. The albino female 

 was later mated with an albino male. Albinos, mated together, 

 never produce any but albino young, a fact well known to all breeders 

 of small mammals. Yet in this case, the young, six in number, 

 were all black. These young were in three litters, born from 6 

 months to a year after the operation. The immature ovaries of the 

 black female were subject to the influence of the blood of the albino 

 for from 4 to 10 months before the egg cells attained full growth and 

 were discharged. Through it all they retained their original heredi- 

 tary potentialities unchanged. 



MODIFICATION OF HEREDITY. 



Although the reproductive cells are not produced by the body, the 

 possibility must be recognized that they may be modified in some 

 cases by substances circulating in the blood. Recent experiments 

 have, in fact, shown that changes can be brought about in the general 

 vigor of the offspring in this way. Dr. C. R. Stockard, of Cornell 

 University, tested the effect of daily intoxication of guinea pigs 

 with alcohol. The animals themselves remained vigorous through- 

 out the treatment. Their young, however, were markedly unthrifty 

 compared with those of an unintoxicated control stock. This was 

 true even when an alcoholic male was mated with a normal female, 

 indicating that the reproductive cells of the male had been damaged 

 by the alcohol. The injury seemed to be permanent, since a second 

 generation produced by first generation animals, which had never 



