30 A STUDY IN HEREDITY 



Accordingly, as diseases resemble in type 

 contagious diseases or malaria, locality is of little 

 or of much importance. Contagious diseases, since 

 they inhabit only the human body, are not affected 

 by locality, and therefore have travelled everywhere, 

 and are now of world-wide distribution. In this 

 they are resembled by consumption, measles, and 

 other earth- and air-borne diseases. Cholera, 

 yellow-fever, etc., because more dependent on the 

 outside world, are of more local distribution. 

 Malaria, which is entirely dependent on the outside 

 world, is strictly local ; it does not travel the 

 world over, but infests certain well defined 

 districts. 



As we have seen, men differ from — are superior 

 or inferior to — their parents and fellows in every 

 respect — in size, in strength, in colour, etc., but, 

 as a rule, they resemble their parents more than 

 they do other men. Thus fair men tend to have 

 fair children, big men to have big children, and so 

 forth. It is a matter of common knowledge that 

 men differ in their powers of resisting this or that 

 disease. Some men take a disease and perish ; 

 others take it and recover ; yet others do not take 

 the disease at all, they are totally immune. It is 

 also a matter of common knowledge that every 

 prevalent disease tends to afflict certain families 

 more than it does others ; in other words, parents 



