l8o BACTERIA. 



that cholera does not show itself during the summer when the air tempera- 

 ture is at its maximum, but only when sufficient time has elapsed to allow 

 of the rising of the ground temperature or that of the earth, and as a matter 

 of fact the disease occurs in Europe during the autumn months often when 

 the general or atmospheric temperature has actually begun to fall, but when 

 the earth temperature has attained its most equable maximum, when of 

 course we have the optimum temperature conditions for the growth of 

 the cholera organism in the soil. " At the mouth of the Nile the con- 

 ditions are much the same as near Shanghai, except that the extremes of 

 heat and cold are greater at the latter. Both have at least weekly communi- 

 cation with India, both communicate through the tropics ; whilst Shanghai 

 is at a distance of at least three weeks in time, Egypt is but two. The 

 regularity of the yearly outbreak at Shanghai points to an endemic poison, 

 the irregularity at the mouth of the Nile to the occasional introduction of 

 the poison. Late summer is also the period for the Egyptian epidemics of 

 any extent." 



Bearing on this, we have the fact that a cholera 

 epidemic may remain dormant for months or a whole 

 winter ; especially as the organisms retain their capacity 

 of reproduction if they are kept moist and are supplied 

 with oxygen. According to Nicati and Reitsch, cholera 

 bacilli were found alive eighty-one days after they had 

 been placed in the harbour water of Marseilles. Koch 

 found cholera bacilli capable of reproduction after they 

 had grown one hundred and forty-four days on agar, but 

 in one hundred and seventy-five days these cultivations 

 were found to be dead. Macleod found that pure culti- 

 vations renewed but once a month retained their virulence 

 for a year. Taking all these facts into consideration, it 

 is easy to understand how in Shanghai the organisms 

 may remain dormant for a certain period, and then under 

 favourable conditions begin to grow again with increased 

 activity. As Koch puts it " One can easily imagine that in 

 superficial layers of earth, in marshes, and so forth, the cholera 

 bacilli may find conditions in which they can exist preserved 

 from death for five months or even longer, just as well or 

 even better than on our moist agar jelly." Here then are 

 three places which, although they have certain features in 

 common, are characterized by certain differences, all of which 

 appear to depend on climatic conditions. In the one case 

 the cholera is prevalent during the whole year ; it is in fact 

 endemic and continuous. In the second case cholera appears 

 with the utmost regularity at certain seasons of the year, and 

 then as regularly disappears. In this case the disease appears 



