140 THE PEINCIPLES OF HEREDITY 



not less marked than his evolution against malaria, but owing 

 to the insidious nature of the former disease, the gradual 

 character of the attack, and the slowness with which symptoms 

 supervene, it is not so striking to the casual observer. In 

 malaria toxins are present in abundance, and are of great 

 virulence, and therefore the person attacked passes in a few 

 hours from apparent health to extreme illness. Within 

 twenty-four hours of entering an infected country he may 

 manifest the symptoms of a virulent seizure. Ships navigated 

 by men of a race which has undergone no evolution against 

 the disease, may have the whole crew stricken down on 

 entering a malarious port, while the natives around retain 

 their health. Invading armies from beyond the borders of 

 malaria have been decimated, and rendered useless as fighting 

 forces, while the inhabitants of the land were able to pursue 

 their ordinary avocations. Moreover, in malarious countries, 

 the pathogenetic micro-organisms are everywhere present, and 

 therefore no susceptible person escapes infection. 



226. But in tuberculosis the toxins are conspicuously feeble. 

 Infection is not marked by sudden and manifest illness. A 

 slow, long-continued "personal" struggle between the phago- 

 cytes and the pathogenetic micro-organisms, which, however, is 

 shorter in the less resistant than in the more resistant, in, 

 generally speaking, the men of a race to which the disease is 

 strange, than in those of a race to which it is familiar. Even 

 in countries where the pathogenetic organisms are most 

 abundant they are not everywhere present, but are more or 

 less limited to crowded and ill- ventilated domiciles to which 

 infected persons have access. Individual powers of resistance, 

 even among peoples to which the disease is quite strange, 

 vary very largely. Parties of strangers from beyond the 

 infected areas are, therefore, never stricken down en masse, 

 but one by one, at different intervals ; and the symptoms 

 noticeable in the sufferers are such as are referable by un- 

 skilled observers to other diseases coughs, colds, and so forth. 

 Last, but not least among the races which are least resistant 

 to malaria is our own ; on the other hand, our race is among 

 the most resistant to tuberculosis ; and therefore our attention 

 is not drawn, in the same marked manner, to racial differences 

 in relation to the latter disease as it is to differences in relation 

 to the former. We have all heard, for instance, of the sufferings 

 from malaria of our compatriots in India and on the West 

 Coast of Africa, and that in the year 1809 a British army was 

 destroyed as a fighting force by the same cause in the Island 

 of Walcheren, but few of us know " that of 9,000 Kaffirs 



