i20 READINGS IN BIOLOGICAL SCIENCE 



obvious Mendelian case a particular character, which to familiar scrutiny 

 is simple and definite, is controlled by the presence or absence of a single 

 inheritable unit known as the gene. Color in animals, eye color in man, 

 taliness or dwarfness in the garden pea are such characters and their study 

 clearly defined the Mendelian principle in inheritance. Skin color in man 

 if albinism is contrasted with the presence of any pigment is similarly con- 

 trolled. 



But skin color among the pigmented of the human species, taliness or 

 shortness in the human race (excepting particular types of dwarfism), 

 the weight or ear length in rabbits and innumerable other conditions are 

 at first sight not so controlled. The result of a cross between individuals 

 of widely diff^erent character is usually a "blended" or intermediate state 

 in the ofl^spring. While it was difficult at first, as has been said, to fit these 

 cases to the Mendelian hypothesis it is now apparent that blended in- 

 heritance means that the character as expressed in the individual is the re- 

 sultant of the combined and overlapping functional expression of the 

 action of two or more genes. It now is the consensus of opinion among stu- 

 dents of heredity that this is the true significance of blended inheritance. 

 Mendelian principles are as strictly appUcable as in the more obvious in- 

 stances but more than one, often many, unit characters are involved in the 

 make-up of the observable quality. This is evidently the condition under- 

 lying the inherited factors in resistance to tuberculosis. 



CANCER AND OTHER MALIGNANT TUMORS 



In general the state of our knowledge of the factors underlying the 

 occurrence of malignant tumors is not dissimlar to that with regard to 

 tuberculosis. The evidence from human sources is of about the same order 

 but less significant on the whole. Tumors have been alleged to frequent 

 occasionally certain families while others remain quite untouched. In the 

 mass there is the sporadic, occasional appearance of a tumor case in most 

 family histories. Cancer is not believed by most authorities to be an in- 

 fectious disease although the fact that it can apparently be initiated in 

 man and animals by chronic irritation with various substances, even by 

 various parasites, creates many resemblances between tumors and infec- 

 tions. If the tum.ors classified as sarcomata are included there are cases 

 in which the utmost consideration of detail fails to reveal any precise 

 reason why they should not be accepted as infections; and yet because 

 of that fact that these appearances suggesting infection are the exception 

 rather than the rule, most scientists hold in reserve the thought that even 

 in these cases it is more than possible that some other explanation will be 

 found, that eventually it will appear that all the true malignant tumors 

 (including most forms now classed as such) will be found to originate 

 in causes resident within the body. 



Tumors bear a certain resemblance to infection in that those which orig- 



