66 Trans. Acad. Sci. of St. Louis. -^ 



The ticks most completely fulfill tliese conditions, and 

 are actually found to have the greatest distribution 

 among the most diversified hosts. 



Any one of the following conditions appears to cause 

 a very restricted host distribution, and in many cases a 

 restriction to a single host species or even to a single 

 host individual : 



Endocolous habits. 



The possession of highly specialized clinging apparatus for holding 

 on to external growths of the host or hosts. 



A delicate and degenerate digestive system (This condition often 

 gives rise to physiological species in forms which are anatomically 

 identical). 



The inability of the parasite to live off its host because of climatic 

 conditions. 



The inability of the parasite to walk when detached from a host. 



The host being one of solitary and monogamous habits. 



As TO THE Analysis and Classification of the Various 

 Types and Aspects of Parasitism. 



Although various classifications of the parasites have 

 been made, and many m^ore will doubtless follow, all of 

 them may be included in three groups: 



Those which take into account the nature and habits of the parasites 

 themselves. 



Those which take into account the nature and habits of the organ- 

 isms attacked. 



Those which take into account the inter-relations of the parasites 

 and the organisms attacked. 



Under each of these three general groups several classi- 

 fications may be made having for a basis either habits, 

 morphological characters, or ecological relations; and 

 these classifications may each in turn be subdivided into 

 others of a more restricted meaning, having for their 

 bases these same factors. 



