6) The species of the parasite transfers to a new host abandon- 

 ing the ancient one completely because of some changes of ecological nature; 



7) The species of parasite transfers to a new host--new hosts, 

 widening its circle of hosts while preserving the old host; 



8) The species of parasites transfers to a new host--new hosts, 

 in connection with the extinction of the old host. 



It is possible that there can be other more complex cases but 

 basically one can consider that the relations indicated fit into the enumerated 

 8 variants. At the same time we do hot enter into the discussion of the 

 causes which provoke certain evolutionary changes in the hosts as well as 

 in the parasites because the discussion of the general questions of the 

 formation of species would have sidetracked us too far from the main 

 theme of research. 



Strictly speaking ,w^e observe phylogenetic parallelism only 

 in the cases anticipated in points 1 and 4; w^hereas, all the rest give a 

 different nature of interrelations between the history of the species of 

 the parasite and the host. From this alone it is apparent that the phylo- 

 genetic parallelism is a particularity of evolutionary changes of the 

 historical pair, parasite-host. However, this perhaps is true only in 

 theoretical conditions, whereas practically, the case of phylogenetic 

 parallelism is most often encountered and by this very fact represents 

 the basic way of evolutionary changes of parasitic animals? A detailed 

 analysis shows that this is not so and that we can with certainty note in nature 

 the presence of evolutionary changes which fit into the theoretical cases p. 299 



indicated in points 1, 2, 3, 4, 6, and 7 and only a few, 5, and 8 ca.nnot be 

 shown in contemporary material. The latter will give us sufficient proofs 

 of more or less particular change along all the possible ways and not only 

 by way of phylogenetic parallelism oi the species. In spite of the fact that 

 the absence of parallelism in the evolutionary development between the 

 species of the parasite and the host is encountered more often than its 

 presence, nevertheless basically the occurrence of parasites on their 

 hosts reflects consanguinous relations of the first and of the second and 

 in a vast majority of the cases among the parasites which actively infect 

 their hosts (see page 289), particularly among monogenetic trematodes. 

 Thus, related parasites with the correlations of the 1st, 2nd, 3rd, and 

 4th types will, as a rule, be encountered on related hosts; in the 5th type 

 the parasite disappears altogether and during the 6th, 7th, and 8th the 

 transfers of the parasite both to related,just as to unrelated, hosts are 

 possible. As the analysis shows in these cases, with the active type of 

 infection we shall have mainly the narrow specificity and the infection of 

 hosts related to the former, and during the passive--a wider specificity 

 and the infection of both the related and just as unrelated hosts, i. e. , the 



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