VARIATION AND HEREDITY 239 



" taint " of which, in popular thinking, is indissolubly 

 linked with the idea of its transmission, is not really 

 inherited, 1 but is in each case a new infection. 



The variation in the degree of specific immunity 

 or resistance to certain diseases may be racial as 

 well as individual. Thus it is frequently stated 

 that various children's diseases, such as mumps, 

 measles, etc., which with us are annoying, but by 

 no means dangerous, affections, when introduced 

 among isolated populations, such as those of the 

 South Sea Islands, develop a virulence unknown 

 among us, and may carry off whole communities. 

 The " resistance " of strains or varieties of plants 

 to disease is quite comparable to that of animals. 

 The inheritance of resistance to the fungus-disease, 

 " rust," to which wheat is subject, has just been 

 mentioned. Without doubt a similar immunity- 

 factor for various human diseases will be discovered 

 in the near future. 



Inheritance of Defects. Nevertheless, observa- 

 tion teaches us that certain human conditions usually 

 classed as diseases are indeed directly inherited. 

 Deafness and color-blindness are found to be trans- 

 mitted in the same way that hair-color or eye-color is. 

 Haemophilia, the name given to the condition in 

 which the blood does not clot and on account of 

 which the victim may bleed to death from a scratch, 



1 "It is quite certain that the children of lepers born out of leper 

 districts, in England or the United States, for example, never inherit 

 it" Quoted by Thompson from Hutchinson in Allbutt's " System of 

 Medicine," I, 1896. 



