HEREDITY 2 2 I 



inate in animals are often transferable to other animals of the same species 

 by a succession of transplantations of the tumor tissue, or in some instances 

 by extracts of this tissue containing no intact body cells. The conditions 

 governing the transplantation are such as to make the influence of in- 

 heritance very apparent. These tumors are never transferable outside the 

 species of animal in which they originate. For instance, mouse tumors can 

 only be propagated in mice. Within the species they are transferable with 

 great difficulty when at all, from one race to another. 



This variation in resistance has been the subject of thorough genetic 

 experimentation and analysis in certain instances. When the Japanese 

 waltzing mouse and the common tame mouse were compared it was found 

 that their differences with respect to tumor transplantability across the 

 race hne must be under the influence of at least twelve separately inherited 

 unit characters. The reasoning applied to the case of tuberculosis in the 

 preceding paragraphs holds here. We should expect the familial evidence 

 for inheritability in the human race to appear only very occasionally. 

 Even less is known about the fundamental nature of the inherited char- 

 acters in tumors than in tuberculosis. 



There is also a great deal of evidence that the incidence of spontaneous 

 malignant tumors in animals is quite dependent on the inheritance. 



DISEASES BASED ON ABNORMAL SENSITIZATION 



A number of disease conditions, all troublesome and some very serious, 

 asthma, hay fever, and various "idiosyncrasies" against particular articles 

 of food or particular drugs have been found to have certain features in 

 common. They are alike in that they are all unusual reactions to partic- 

 ular substances found in the environment which do not affect most people 

 in any harmful way. Of those suffering from the condition some react 

 only to a single substance, others are affected by many substances. The 

 diseases are so common as to be familiar to most people and place need 

 not be given here to any detailed description of them. The simplest, and 

 in many ways most characteristic, is urticaria, or hives. Most people suffer 

 at one time or another from this trouble. Some people always have it as a 

 consequence of eating a particular food, e. g. strawberries, eggs. The skin 

 becomes blotched and irregular wheals are raised above the general level 

 of the skin surface by reason of the fact that the skin in these areas is 

 swollen. The sweUing is due to fluid in these areas having left the blood 

 vessels and stagnated in the tissue spaces. In asthma the same general proc- 

 ess occurs but the area affected by the swelling is the smaller air tubes 

 in the lungs and these are partly closed, making breathing difficult. In hay 

 fever it is the mucous membranes of the eyes and nose which are affected. 



Inquiry has disclosed a well-marked familial influence in these condi- 

 tions. They are in some measure inherited. The inheritance seems to be 

 based on recessive characters in the Mendelian sense. There is a certain 



