352 



contact with or feeding from his discharges), but may be likewise inherited 

 by the nest generation of mosquitoes issued from the contaminated parent. 

 With this new development, indeed, the theory seems to cover the whole 

 ground of conditions which are known to govern the transmission or non- 

 transmission of the disease, and to account for every well authenticated 

 fact on record. 



There are different species of mosquitoes which are peculiar to certain 

 localities, even within the yellow fever zone, though we are not able to 

 account for their preferences in this respect, nor for the circumstances 

 which determine their permanency in such limited areas. It is a fact, how- 

 ever, that the ' ' domestic mosquito " (by which term I mean such species as 

 constitute a parasite and comensal of man) shows no inclination whatever 

 for extensive excursions so long as the female insect finds at hand conve- 

 nient subjects from whom they can draw the warm life-blood which they 

 apparently require, not so much for their own nourishment as for the 

 purpose of ovulating and for the reproduction of their species. That the 

 tropical species cannot well establish themselves in temperate or cold clim- 

 ates, will be readily understood by repeating the following experiments : 

 Let a mosquito of the small diurnal species found in Havana be introduced 

 into a test tube provided with a thermometer; if the temperature be low- 

 ered, it will be seen that, when it falls to between 15° and 19° C. (59° 

 and G2.2° P.) the insect becomes benumbed, and unless it finds some ob- 

 ject upon which it can fasten its claws it will drop to the bottom of the tube, 

 in a condition of apparent death, in. which it will remain so long as the 

 temperature is kept between 15° and 0° C. (59° and 32° F.) On allowing 

 the temperature to rise again, when it reaches above 15° to 19° C. the mo- 

 squito will gradually revive and soon recovers its normal agility and the 

 power of stinging. On the other hand, if the mosquito is confined in a closed 

 tube and the air gradually rarefied, the insect appears to lose the power 

 of supporting itself on its wings, and also that of stiffening its proboscis 

 for the purpose of stinging. With temperaturesi below 25° C. (77° 

 F.) the time required by eulex mosquito to digest the blood and get ready 

 for another bite is prolonged to several days; and, according to my 

 observations, the pupae of that species, if kept at temperatures 23° C. give 

 five males to one female, whereas at 25°-30° C. the proportions are reversed. 

 Hence it follows that during the cold season in Havana there are com- 

 paratively few mosquitoes of that kind in a condition to propagate the 

 yellow fever. 



At sea mosquitoes will save themselves from being blown into the 

 water only by avoiding the deck, and other exposed parts of the vessel, and 

 in travelling towards cooler regions they will have an additional motive 

 for seeking refuge in the warmer and more sheltered parts of the ship. 

 Hence the likelihood of their gathering in the holds of vessels, in which 

 the source of yellow fever infections has been many a time distinctly 



