32 Heredity. 



necessarily continued to succeeding generations. Only such con- 

 ditions are transmitted as pre-existed in the germ, in the seg- 

 mentation cells and the embryo, or those in which apparently the 

 nature of the whole parental cellular structure is altered, as 

 germinal variations and so-called constitutional tendencies and 

 diseases. 



Injuries and diseases which do not involve the sexual cells, as 

 all simple traumatic lesions of the body, give rise to no trans- 

 mission. For example, the common custom of cropping the ears 

 and tails of dogs has never led to the birth of a short-eared or 

 stub-tailed dog or of such a breed (just as the practice of cir- 

 cumcision, practiced for hundreds of years among various peo- 

 ple^ has never yet caused any congenital anomalies of the 

 prepuce). If occasionally structural anomalies of this type are 

 encountered careful investigation will show that some intrauterine 

 mechanical fault is responsible for the defect in the foetus, as 

 Bonnet has demonstrated in case of stump-tailed dogs that the 

 caudal vertebrae have been bent or deformed simply by intra- 

 uterine pressure (amnion). This is also proved by the fact that 

 the same anomaly occurs in cats (and in any other animals whose 

 tails are unobjectionable to man and for the removal of which 

 there is no occasion), showing that the habit of cropping can 

 scarcely be held responsible. 



Non-transmissible conditions and those which are truly in- 

 herited may very closely resemble each other in their anatomical 

 features, and yet depend on entirely different causes ; only the 

 closest inquiry revealing the fundamental influences producing 

 them. For example, supernumerary toes (polydactylism) may be 

 an atavistic phenomenon, a. family trait, or may occur in a foetus 

 as a splitting of a digit because of amniotic adhesions ; in the 

 same way harelip (congenital fissure of the Ijp) may be caused 

 by local amniotic trauma or may be an hereditary anomaly. 



Heredity is best understood when it concerns chemical in- 

 Hucnces. As indicated by Ribbert, it may be conceived that the 

 whole bodv can suffer from the wide dissemination through it of 

 some chemical substance and that, of course, under the circum- 

 stances the germ cell is also reached by the same noxious ma- 

 terial. If the organism survive such disturbance and become 

 immune through the changes called forth by the chemical poison, 

 the germ cell may also survive with the same iniinitiuciiig changes. 

 Should such a process befall both parents, both spermatozoon 



