June, '08] JOURNAL OF ECONOMIC ENTOMOLOGY 215 



the blood of the host were transmitted by the ticks to other animals 

 of the same nature without serious danger. But when a tick contain- 

 ing the blood parasites of one, its natural, host becomes attached to 

 a new and different kind of host, then the blood-parasite in this alien 

 blood may originate a disease. The occasional transference of a 

 tick from one host to another may not be sufficient, but when a species 

 of tick practically changes its host, then a disease may result, pro- 

 vided, of course, that the ticks are commonly infected with a blood 

 parasite of their old host. 



This theory of the origin of these diseases, though new to me, I 

 find has been proposed by Dr. H. M. Woodcock in a paper on the 

 Hsemoflagellates.* Doctor Woodcock was mostly concerned in the 

 diseases transmitted by flies, but as he includes in his general review a 

 reference to piroplasmosis, it is evident that he considers the tick- 

 borne diseases as originating in the same way as the others. Doctor 

 Woodcock's statement follows: "It follows, however, from what 

 has been said above, that the animals for which these parasites are 

 markedly pathogenic cannot be regarded as their true or natural hosts, 

 which are rather to be sought among the native, tolerant animals of 

 the locality concerned." 



In accordance with this theory then, the ticks in adapting them- 

 selves to the march of civilization, the extermination of native ani- 

 mals and the introduction of domestic animals, have here and there 

 transmitted to domestic animals blood-parasites that are normally 

 found in certain wild species. 



The tick is a most necessary part in the life-history of these para- 

 sites, for in some cases (perhaps all) the sexual conjugation of the 

 parasite is consummated within the body of the tick. 



It is therefore evident that all ticks are potentially dangerous. 

 Any tick now commonly infesting some wild animal, may, as its 

 natural host becomes more uncommon, attach itself to some domestic 

 animal. Since most of the hosts of ticks have some blood-parasites, 

 the ticks by changing the host may transplant the blood-parasite into 

 the new host, producing, under suitable conditions, some disease. 

 Numerous investigators throughout the world are studying this phase 

 of tick-life, and many discoveries will doubtless signalize the coming 

 years. 



* Quart. Journ. Micr. Sci. (N. S.) vol. 50, p. 158, 1906. 



