146 THE PRINCIPLES OF HEREDITY 



237. It is clear, therefore, that the individuals of our race 

 are very generally so resistant to tuberculosis that even after 

 infection their phagocytes, under slightly improved conditions, 

 are able to wage successful war against the microbes. It is 

 clear also that no immunity can be acquired against the 

 disease, since those who have recovered may, under worse 

 conditions, take it again and again. Their only safety lies in 

 the absence of the pathogenetic micro-organisms. Thousands 

 of our race who are unable to resist the attacks of tuber- 

 culosis in their native land, and on that account are obliged 

 to leave it, are able to maintain a healthy existence under 

 conditions that yet prevail in all such parts of the New 

 World as have not long been settled by us in certain parts of 

 America, in Australia, in New Zealand, in the Pacific Islands, 

 and also in South Africa. But in these very lands, where the 

 less resistant among us recover from previous infection 

 tuberculosis is causing the extermination of the natives. 

 This one fact throws the greatness of our evolution into 

 startling relief, for the natives usually live under hygienic 

 conditions that are far better as regards the disease than do 

 the settlers. The latter endeavour to reproduce their home 

 life as nearly as possible ; they gather themselves into urban 

 communities, and build much the same kind of houses as 

 those in which they contracted the disease; whereas the 

 natives dwell scattered, or at worst in small communities, 

 and in dwellings more wind-swept than the shanties of the 

 Hebridean fisher-folk. Nevertheless they perish and their 

 races are becoming extinct : for so susceptible are they that 

 they take the disease in circumstances in which the most 

 susceptible Europeans live immune ; and they are so little 

 resistant that they take it in its most virulent form. The 

 microbes, unchecked by the phagocytes, multiply within them 

 at a rapid rate, they exhibit all the phenomena of galloping 

 consumption ; and even in their draughty wigwams and 

 whares they infect their fellows. To infect a normal European 

 a considerable dose of the virus seems necessary, since so 

 many of the parasites succumb in the struggle with the phago- 

 cytes ; to infect a Red Indian or a Maori the smallest possible 

 dose seems sufficient, since the phagocytes seem to have no 

 power of destroying the bacilli. 



238. " Consumption is prevalent to a most disastrous ex- 

 tent among the races of the Southern Pacific. We have more 

 particular accounts for Fiji and Tonga, Samoa, Tahiti, the 

 Marquesas and Hawaii (Honolulu). In New Caledonia the 

 death-rate from consumption among the Kanakas is estimated 



