peculiarities - -feeding at the expense of the host and the appearance of 

 morphological adaptations for attachment to the constantly moving body of 

 the host. It seems to us that these two processes are interrelated and in 

 certain measure interconditioned. The factor which was favorable to the 

 contact between the ancestors of monogenetic trematodes and their hosts 

 was the wide distribution of ancient Selachii in the shallow fresh waters 

 where the ancestors of Dalyellidae mainly lived and where the possibility 

 of mutual occurrence was very great. Theoretically we can represent the 

 progress of changes of the degree of the first wide specificity in the 

 following directions; (1) preservation of wide specificity and (2) gradual 

 development of more and more narrow specificity. It is understandable 

 that with this, complicating factors may also occur, that is, the secondary 

 development of wider specificity and the preservation of a determined 

 degree of specificity for a long time. 



Thus, we approach the question about the correlation between p. 292 

 the phylogeny of the parasites and the phylogeny of their hosts, directly, 

 i.e., (directly?, nobis) to the so-called question about conjugate evolution 



w^hich is the subject of very lively disputes. By conjugate evolution at the 

 present time is understood a prolonged process of common historical 

 existence of the species of the parasite and host during which changes of 

 the parasite take place simultaneously, or almost simultaneously , with the 

 evolutionary changes of the host on or in which it lives. Hence, it is supposed 

 tliat consanguinous hosts contain related parasites and the phylogenetic relations 

 of the latter reflect corresponding changes among the former, that is, there is a 

 so-called phylogenetic parallelism between the hosts and their parasites. 



Without any doubt the presence of phylogenetic parallelism, in 

 addition to being of considerable interest in itself, would have given into 

 the hands of the researchers a very valuable method of study of various 

 questions connected to the problems of evolution of parasites, their hosts, 

 zoogeography and so forth. However, in order to approach this it is 

 necessary to clarify whether or not actually there is a conjugate evolution 

 of the parasites and their hosts, what is the scope of distribution of this 

 process, whether it is observed, and what are the causes which produce 

 deviations and exceptions from the general rule. 



Considerable literature abroad and especially in the Soviet 

 Union is dedicated to the questions of phylogenetic parallelism and the 

 problem of conjugate evolution. We will not dwell especially on it, 

 referring those who are interested to the sources (Ass, 1939; Rubtsov, 1940; 

 Eichler, 1940; Kirschenblat, 1941;Szidat, 1944; Markov, 1948 and 1954 

 and others), but we must analyze certain questions stated in this literature 

 because they are closely related with the problems interesting us. 



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