272 THE PRESENT EVDLUTION OF MAN — PHYSICAL 



organisms, which arc, however, destroyed by the phago- 

 cytes before they are able to multiply and produce the 

 disease. Thus small doses of toxins are introduced into 

 the system, in response to the stimulation of which the 

 phagocytes react to some extent, and attain a position 

 of some advantage, which, however, since the stimula- 

 tion is so much less, is not so great or so permanent as 

 that which results in an individual who has actually 

 experienced and recovered from the disease. Such 

 powers are therefore qiiite lapsed during residence in 

 a non-infected area, and therefore an individual who 

 returns from a lengthened sojourn in it is liable to fall a 

 victim to the disease to whicli he was formerly immune. 

 A third point of interest is the following : diseases 

 in which the toxins are powerful are usually exclusive 

 each one of all others ; that is, such diseases seldom, if 

 ever, co-exist in the same individual. The reason for 

 this must be that the toxins of a disease that has 

 established itself, pt)isonous as they are to the com- 

 ponent cells of the host, are equally or more poisonous 

 to the microbes of other diseases, which therefore are 

 unable to multiply in such an unfavourable environ- 

 ment. Probably these low unicellular organisms are 

 not able to vary in a fit direction like the cells of high 

 multicellular organisms, and for that reason probably it 

 is that we sometimes hear of a long-established disease, 

 — c. g. lupus, — the toxins of which are not virulent, 

 beinfT cured or abated after the host has suffered from 

 some other disease, the toxins of which are virulent — 

 c. g. erysipelas. In such a case the toxins of the latter 

 must be even more effective against the microbes of 

 the old-established disease than against the phagocytes 

 of the host ; must even more enfeeble them than the 

 phagocytes; in consequence of which, when recovery 

 from the new disease occurs, the old disease is also 

 conquered. 



