PAPERS ON BIOLOGY AND AGRICULTURE 137 



free existence is frequently during the egg or larval 

 stages of the parasite, but in the hair-snakes (Gordiacea) 

 the mature worms are free-living and produce their eggs 

 while living free in the water. An environment lacking 

 water offers obstacles to the development of the Gor- 

 diacea, for the worms leave the bodies of their insect 

 hosts when the insects fall into water. 



The hookworm of man (Neeator), which causes such 

 heavy economic losses to the region of infestation in the 

 South, offers no problems in our latitude. Even hook- 

 worm sufferers introduced here could not become sources 

 of serious infestation to others because of limiting fac- 

 tors which are operative. The eggs leave the body of the 

 host along with fecal matter and live for some time in 

 the polluted soil. In this free stage the larvae are unable 

 to withstand freezing temperatures, so in this latitude 

 persons could not be exposed continuously to sources of 

 infestation such as exist in the South. Furthermore, im- 

 proved sanitary conditions do not permit contamination 

 of the soil where human beings would come into contact 

 with the infecting larvae. 



Many trematodes, which produce diseases in man and 

 in other animals, are restricted in their distribution 

 to regions where suitable larval hosts occur. Since moll- 

 uscs are the usual larval hosts of trematodes and since 

 most of the molluscs are aquatic, presence of waterbodies 

 is favorable toward trematode occurrence, while lack of 

 water minimizes the possibilities of trematode infesta- 

 tions. 



Many other animal parasites are so affected by condi- 

 tions external to their hosts that they are excluded from 

 certain areas wherein physical conditions suitable for 

 their development are lacking. 



ABSENCE OF ESSENTIAL LARVAL HOSTS 



When a parasite which involves an alternation of hosts 

 in its cycle of development invades new territory which 

 is not occupied by one of its hosts, it fails to become es- 

 tablished in the fauna of that region unless it is able to 

 acquire as a host some other form of life represented in 

 the local fauna. Both malaria and yellow fever are pro- 



