finds its reflection in the possibility of cross -hybridization of the hosts or 

 in their consanguinous relations. At the same time , ecological factors 

 obviously play a secondary role. Thus, the same species - -Dactylogyrus 

 difformis Wagener, is encountered mainly among fishes feeding predomi- 

 nantly on vegetational food [Scardinius e ry throphthalmus (L. )] , but simul- 

 taneously and among typically benthopelagic fishes feeding mainly on 

 benthos [Abramis brama (L. ) and others] and maintaining themselves near 

 the surface of the water and feeding on plankton and air insects [Alburnus 

 alburnus (L. )] . In D. similis Wagener, for instance, we find fishes with 

 different biology [for instance Abramus brama (L. ) and Chondrostoma 

 nasus (L. )] among its hosts. The number of similar cases can be con- 

 siderably increased if desired. If this is so, and we have no basis to 

 doubt it, then it is most probable that in the foundations of "the rule of 

 Fuhrmann" lie deeper physiological reasons which find their reflection 

 in natural taxonomic classifications of the host. Beyond any doubt 

 ecological factors played and are playing a considerable role in the formation 

 of these physiological properties and peculiarities, but often already in 

 "skimmed" (reduced, nobis) form. Hence >it is clear that we think that 

 "the rule of Fuhrmann" reflects those ecologicophysiologlcal peculiarities 



of the organisms which were formed during a prolonged historical period 

 and which at the same time were expressed also in the natural system of 

 the host (that is in their evolutionary development and on the nature of the 

 infection of the latter by the separate species of the parasites). Thus>it is 

 not "the mysterious phylogenesis" (Markov, 1948) which determined the 

 distribution of separate species of parasites on their hosts, but both phylo- 

 genesis and the distribution of parasites reflect complex historically- 

 formed interrelations between the medium and the animals --hosts, and 

 also between the first, the second, and the separate species of the parasite. 

 However, we reiterate that we are only approaching the questions of 

 phylogenesis of the hosts and of their parasites. 



As is apparent from the above-mentioned, "the rule of 

 Fuhrmann" is of rather limited character and can serve in a very limited 

 degree for the purposes which interest us, that is, for the formulation of 

 the natural system of the monogenetic trematodes. Basically, it only helps 

 us in the sense that it enables us to pay attention to the ^,c^^sc xn th^ event 

 of finding a determined species of the parasite on species of the animals 

 which are distant from the usual hosts and, at best, to show that there is 

 an error in determining (the determination of, nobis) the parasite or the 

 host, or that the distant host actually has certain consanguinous relations 

 with the usual hosts. However, properly speaking, that is all that this 

 rule gives us, although in other connections, partly for zoogeographical 

 problems, it is undoubtedly very valuable. 



However, besides "theRule of Fuhrmann" there are other 

 principles (normalities, regularities, nobis) which have great relation to 

 the question which interests us about the correlations of the phylogenesis 



343 



