EVOLUTION AGAINST DISEASE 145 



are leading an active life in the open air, on the march, or 

 in manoeuvres and campaigns ; that the cases mount up as 

 soon as the troops enter on their garrison life, as, for example, 

 in winter, and spend their time in ill-constructed, crowded, 

 filthy, and badly-ventilated barracks. Welch, who treats of 

 this matter with reference to the British Army, says that 

 ' nearly half of army consumption is connected with vitiated 

 barrack atmosphere/ a similar conclusion having been ex- 

 pressed by earlier writers, as Tulloch and Maclean, the latter 

 including in his statement the British and native troops in 

 India. With respect to its frequency in the French army we 

 find a similar reading of the facts in the papers by Champ- 

 pouillon, Tholozan, Viry, Lausies, and others." 1 



236. The conditions under which we live are such that, 

 normally, every individual amongst us is exposed to infection 

 that is, we all at one time or another enter the rooms 

 inhabited by tuberculous sufferers ; but such has been the 

 evolution of resisting power in our race that only about one- 

 eighth of us perish from consumption, while seven-eighths of 

 us live immune to the disease, or recover from it and die 

 from other causes. A notable fact in this connection is the 

 often evenly-balanced nature of the struggle between the 

 microbes and phagocytes. Many of us are resistant under 

 almost any circumstances; but many others exist in the 

 border space between immunity and susceptibility. Such 

 people when their vitality is lowered, or under other circum- 

 stances favourable to the bacilli, contract the disease, but 

 when their vitality is raised, or when the environment becomes 

 less favourable to the pathogenetic micro-organisms, defy it, 

 and, if they are already diseased, recover. Even when they do 

 not recover the disease usually runs a prolonged course in 

 them ; there is a lengthened struggle between the parasites 

 and the phagocytes, and we then behold the phenomena of 

 chronic phthisis. Laennec said that a patient does not die 

 of his first attack of tuberculosis ; that is, a patient of the 

 highly-resistant type which is normal in our race. Less 

 resistant individuals, who have lapsed back to the ancestral 

 condition of greater susceptibility, undoubtedly do often perish 

 of their first attack ; as, for instance, of acute phthisis (gallop- 

 ing consumption) or tubercular meningitis ; but the very 

 general prevalence of more resistant individuals, who, as 

 regards their powers of defence against the bacilli correspond 

 to a far later stage of the life-history, justifies Laennec's 

 dictum. 



1 Hirsch, vol. iii., pp. 221-2. 



