infection, historically the process of establishment of specificity in this 

 case took place, as Dogiel correctly considers it, in the course of general 

 parasitology by means of a gradual contraction of the initial wide circle 

 of hosts in connection with the gradual adaptation to a larger or smaller 

 uniformity of food. 



We now pass to the questions which were stated earlier about p. 291 

 the historical origin of the pair, parasite -host, and about the origin of 

 specificity of the parasites toward their hosts. Answering the first one 

 can say that for the analysis of the historical origin of the pair, parasite - 

 host, it is indispensable to approach the groups which have different ways 

 of infection or in other w^ords, in which the origin of parasitism proceeded 

 differently from different primary peculiarities of their biology. With this, 

 it is indispensable to take into consideration "the triad of Pavlovsky, " in 

 relation to the formation of the pair, parasite -host, with those remarks 

 which have been expressed earlier. This triad shows the final results of 

 the history of the origin of the pair, but at the same time reflects not only 

 the contemporary process, but also provides the key to the understanding 

 of the history. Proceeding from what has been said before, further dis- 

 cussion of the question must of necessity be transferred from the realm 

 of generalities to the concrete terrain of the study of the determined group, 

 parasite -host, in our case --to return to monogenetic trematodes and 

 their hosts. The second question that was raised before is closely linked 

 with the first and, if it can be answered immediately in a general form 

 that specificity arises simultaneously with the process of appearance of 

 pair --parasite -host, then the working out of the means of further changes 

 of specificity, as it seems to us, can be also solved only by the analysis of 

 concrete material concerning the specificity group. 



The historical origin of monogenetic trematodes, as will be 

 analyzed further in detail (see page 323 ), is more probably connected with 

 the period of the separation of fishes as an independent group of classes 

 and consequently the first biocoenotic pairs were, in our case, their 

 ancestors and some sort of ancient fishes, most probably relating to the 

 group Selachii. Without entering at the present time into the question of 

 the origin of monogenetic trematodes. we can consider that ectoparasitism 

 is a primary phenomenon of this group and is characteristic for it from 

 the very beginning of its origin. Thus, the transfer from the free way of life 

 to parasitism characterizes the peculiarities of change of conditions of 

 existence, not of Monogenoidea but of their ancestors. At the same time, 

 undoubtedly they were some sort of dalyellid -like, straight-intestined 

 turbellarian which led a predatory type of life and which changed to the 

 moving habitat on the surface of ancient fishes for feeding on different 

 small invertebrates and perhaps on seaweeds on the latter. Beyond any 

 doubt, this process began often at that time and very wide specificity of 

 these ancestral forms took place. The development of monogenetic 

 trematodes at. a group could proceed along the gradual evolution of two 



335 



