RELATION OF NATURAL TO ALTERNATING PARASITES 743 



ing the larvae of the second generation. Spirochaeta culicis and the 

 spores of gregarines may find their way into adult mosquitoes by a 

 similar route. The spirochaetes and flagellates of fleas also pass through 

 the larval stage of the host. 



Hereditary infection occurs among natural parasites as in those with 

 alternate hosts. At least three instances are known in blood-sucking 

 arthropods, Crithidia melophaga in Melophagus ovinus, a pupiparous fly, 

 Crithidia christophersi in Rhipicephalus sanguineus, and Crithidia 

 haemaphysalidis in Haemaphysalis bispinosa. 



In view of the wide-spread occurrence and practical importance of 

 these natural parasites it will be of interest to consider briefly their 

 origin and their relation to the pathogenic forms which have a second 

 phase of development passed in the vertebrate host. In the earlier 

 chapters special stress has been laid on the comparative anatomy of the 

 mouth parts of the blood-sucking forms, with a view to making it clear 

 that the blood-sucking proboscis is nothing more than an adaptation 

 of the primitive arthropod structures. The remote ancestors of these 

 forms were in fact provided with mouth appendages adapted, as they 

 are in the vast majority of the modern arthropods, for cutting and 

 tearing, and the food absorbed was solid or semi-solid. Now it is known 

 that almost all collections of water, especially if permanent and rich in 

 organic matter, contain large numbers of primitive organisms; what 

 is the case now was presumably the case at the period when a much 

 larger proportion of the surface of the earth was covered by water 

 than is at present, and it follows that the primitive arthropods were 

 necessarily to a large extent aquatic, as so many of them still are in the 

 larval stage. Protozoa of all kinds would be taken up with the food, 

 and in course of time some of these would become adapted to live and 

 multiply within the body of the arthropod. Once this commensal exist- 

 ence were established, the parasite would share in all the influences to 

 which its host was subjected, and its evolution would proceed part passu 

 with that of its host in accordance with changed conditions. 



The occurrence of natural parasites, perfectly harmless to their hosts, 

 in a large proportion of the modern insects is therefore neither very 

 remarkable nor difficult to understand. The question of their relation- 

 ship to the parasites which alternate between two hosts is a more difficult 

 question, and here we are on very debatable ground. Minchin, to whose 

 opinion due weight must be given, is inclined to believe that those 

 haematozoa which have alternate hosts are primarily parasites of the 

 vertebrate, and would even go further, and maintain that the various 



