INFLUENCE OF SEX AND OCCUPATION. 163 



as it is abstinence and not nationality which protects the Europeans 

 in Abyssinia from tape-worm (p. 156). 



In judging such cases, we must, above all, remember that the fre- 

 quency of the occurrence of intestinal worms is determined primarily by 

 the opportunities for the transmission of the emtoyos or larval forms. 

 Customs, habits, occupation, and mode of life deserve the first con- 

 sideration, and have a much greater influence on the occurrence and 

 distribution of parasites than age, sex, or nationality can ever have. 

 After what we have said, it may indeed appear as if the etiological 

 import of the latter factors were after all only apparent, or at least 

 dependent on the numerous inter-relations between them and the 

 true causes. 



What we have emphasised above explains why the appearance of 

 certain forms of helminthiasis is often in striking dependence on condi- 

 tions of time and space. 



In the first connection we have as yet but little material as far as 

 man is concerned, and what we have is uncertain, in so far as many 

 parasites have a somewhat long life, and do not at once attract 

 attention. We may, however, advance thus much, the thread-worm 

 is said to be most common in autumn, 1 as we might indeed expect 

 from its rapid growth already mentioned, whilst Tcenia solium, on 

 the other hand, according to the results of professional helmin- 

 thologists, calls for treatment more frequently in summer, which leads 

 us to regard the infection as having taken place in winter, since the 

 tape-worm requires several months before it makes itself noticeable 

 by the expulsion of proglottides. Indeed, one can hardly doubt that 

 it is in the winter months that the increased consumption of flesh, 

 and the custom of killing animals used for food at home, afford 

 specially favourable opportunities for infection with bladder-worms. 

 For similar reasons Trichina, with only a brief period of incubation, 

 is far more frequently observed in winter than in summer. 



The evidence furnished by the worm diseases of our domestic 

 animals is still more convincing. These occur in most obvious de- 

 pendence on certain periodically recurrent causes. The sheep-cough 

 (Strongylusfilaria), which attacks our flocks in late autumn, may with 

 certainty be said to arise from the meadow pastures, just like the 

 Coenurus, which appears mostly at Christmas, or like Echinorhynchus 

 yigas, which only appears in those swine which have been fed in the 

 open air. Even our geese are infested with worms (especially with 

 Tcenia lanceolata) only while they are seeking their food through 



1 According to Gribbolim, the maxiimun of cases of Ascaris occurs in February (loc. 

 cit., p. 9). In the same way, Oxyuris is most prevalent in January, and Trichocephalui 

 in April. There are fewest cases in August, October, and November. 



