of the hosts with those of their parasites. Without entering into details we 

 must note that conjugate evolution, to a certain degree, or to be more 

 precise- -duration of the process--always takes place because it is impossible 

 to visualize the historical development of the parasite without the host, 

 independently of how long and in what ecological conditions the pair, parasite- 

 host exists. The question about phylogenetic parallelism is another matter. 

 First of all, in the latter case such a period of time of mutual existence of p. 298 

 the pair, parasite-host, is presupposed during which parallel changes of 

 both species or the change of one of them (host) with the preservation in 

 him of the connection of the unchanged other (parasite) are possible. Con- 

 firmations favoring such relations among different groups of parasites are 

 cited in a number of works; particularly the article of I. A. Rubtsov (1940) 

 contains considerable material. For monogenetic trematodes at the pre- 

 sent time the cases with interrupted distribution of species of parasites 

 and hosts cited in our works (Bychowsky, 1948; Bychowsky and Poliansky, 

 1953) and also with authentic data about the changes of species of the 

 parasite following historical changes of the species of the host in connection 

 with the falling of the last into new ecological conditions (Bychowsky, 1949, 

 1951) are convincing examples. Theoretically one can represent the following 

 cases of historical interrelations of the species of the parasite and the host: 



1) Just as the host itself, the species of parasite living on 

 some hosts does not undergo any revolutionary changes during a specific 

 (or limited, nobis ) historical time; 



2) The species of parasite changes evolutionarily, changing 

 into another species or species (pi. ) on the same unchanging species of 

 the host; 



3) The species of the parasite is preserved during the evo- 

 lutionary change of the species of the host which diverges giving rise to 

 new species; at the same time the ancient species of the parasite can occur 

 on one or several of the species formed; 



4) The species of the parasite parallels, more or less, 



(Is synchrnnouo with) the change of the host; at the same time a new or 

 several new species of the parasite occur in different combinations on the 

 newly fornned host species: i.e. , either on each of the new resulting 

 species of the host there will be a new species of parasite or in part (of 

 the hosts, nobis) there will be the same new species of the parasite and 

 so forth; 



5) The species of the parasite becomes extinct during the 

 prolongation of the existence of the host or during its extinction. 



344 



