INFLUENCE OF CLIMATE. 167 



rise above 20 C., or only attains it for a short time. It is all but 

 unknown in Iceland (according to Krabbe), although distributed 

 throughout the Temperate and Torrid Zones. Trichocephalus has a more 

 restricted distribution, since its eggs require still greater warmth. If 

 the temperature required for hatching were in itself decisive, Oxyuris 

 (with a requisite temperature of 38 C.) ought hardly to be found outside 

 the tropical countries, while in fact it is extraordinarily abundant in 

 the far north, and more widely distributed there than elsewhere 

 (Ohlrick). This cosmopolitan distribution is explained when we 

 consider that the aboye-mentioned high temperature is only needed 

 for a few hours to develop the embryos. The warm skin of man 

 affords the requisite conditions of development equally well among 

 the Esquimaux as among the natives of tropical climes. The Oxyuris 

 patient thus bears the means of infection on his own person trans- 

 mission to the mouth is a simple matter the easier, the less the 

 attention paid to the cleanliness of the body and clothing. 1 



We hardly need to discuss how the distribution of Helminths is 

 affected by the habits of the intermediate hosts. Where they are 

 absent there can obviously be no parasite. 2 Parasite and intermediate 

 host are in their occurrence as closely bound up with one another, as 

 Herbivora and the plants upon which they feed. 



We cannot doubt that in the course of time, when our knowledge 

 of the life-history of human intestinal worms is more complete, we 

 shall be able by the discovery of the intermediate hosts to explain the 

 causes of the more or less restricted distribution of some of these 

 parasites. It is especially in regard to the as yet but little known 

 tropical Entozoa that we look for further progress in this direc- 

 tion. The only tropical worm whose intermediate host we know 

 is Filaria Medinensis, which passes its larval life in animals 

 (Cyclops) which are among the most frequent inhabitants of our 

 fresh water. This seems hardly to justify the hope expressed above, 

 but we must remember that, besides the factors emphasised above, 

 many chances of the most diverse kind influence the occurrence of 

 parasites. 



It not unfrequently happens that even when the conditions 

 of occurrence are present, the species occur in one place abun- 

 dantly, but are entirely absent even in the near neighbour- 



1 It is therefore not so surprising that Oxyuris occasionally occurs in sucklings. Grib- 

 bohm saw them in infants five weeks old, but Ascaris and Trichocephalus only in the 

 eleventh month (loc. cit. t p. 6). In contradiction to this statement, however, we read in 

 Goze : " We find instances of large thread -worms voided by infants hardly a month old, 

 who have tasted only their mother's or nurse's milk " (loc. clt., p. 66). 



2 Thus, e.g., Taznia serrata, so common in Germany, is not found in Iceland, where 

 there are no hares or rabbits (Krabbe). 



