PARASITES AND THEIR DEVELOPMENT. 203 



related but different species inhabiting another area of the 

 world. Regarding the case of the parasites in this light, we 

 may deduce a similar conclusion, namely, that the " broad- 

 head" may discharge in its especial field of action the functions 

 performed in other fields or areas by the common tapeworm. 

 Nature, in any case, may certainly be credited with the 

 general avoidance of any confusion of interests, and with the 

 exclusion of rivalry from the domain and functions of like or 

 nearly related creatures, wherever that domain may exist, and 

 whatever these functions may be. 



As a final example of a most singular and at the same 

 time utterly harmless little intruder on the human 

 domain, may be mentioned the minute mite known 

 to naturalists as a species of Demodex (Fig. 26), 

 and which, curiously enough, seems to take up its 

 abode in the ducts or " follicles " of the skin at the 

 sides of the nose. It is highly probable that this 

 little creature is very frequently to be found in the 

 situation just mentioned, its minute size and harm- 

 less character preventing our being made aware 

 of its mere existence. Demodex measures a mere 

 fraction of an inch in length, and may be said to 

 present us with yet another instance of an organism 

 whose selective powers in the choice of a habitation appear 

 to be of the most singular description. 



The lessons to be drawn from a consideration of the 

 entire subject of the parasitic enemies of man bear very 

 strongly on questions of common hygiene and sanitation. 

 The extension of our knowledge of parasites and of their 

 life-histories clearly points to the desirability for the exercise 

 of great care in the choice and preparation of our common 

 foods, especially of animal kind. Uncooked animal food 

 in any form should be unhesitatingly rejected on common 

 sanitary grounds, the prevailing and fashionable taste for 

 "underdone" meat notwithstanding. The Mosaic abhor- 

 rence of the pig is fully justified by an appeal to zoological 

 knowledge regarding the parasites to which that familiar and 



