CHAPTER VI 



CERTAIN GENERAL CONSIDERATIONS ABOUT OCCURRENCE 



AND SPECIFICITY 



AH that has been said above concerning the occurrence of p. 283 



monogenetic trematodes on their hosts must be subjected to further dis- 

 cussion in order to deternnine not only the facts of finding certain species 

 and higher taxonomic groupings of parasites on particular systematic groups 

 of the hosts, but also to what degree the normalities (principles or regu- 

 larities, nobis) thus obtained and their exceptions can be utilized in order 

 to understand the historical correlation between the parasites and their 

 hosts, and for the formulation of the systematics of monogenetic trematodes, 

 i.e., the main problem which stands before the present research. 



However, first we must consider the general questions connected 

 with the so-called problem of specificity for, as every reader understands, 

 everything which has been said before is directly related to this question. 



We have already often used the term "specificity" without, how- 

 ever, explaining what is meant by it. Stiange as it may seem there is not 

 a single clearly expressed definition of specificity; moreover, the majority 

 of the authors who propose a new definition state, at the same time that 

 other definitions are either inaccurate or even erroneous and methodolo- 

 gically unacceptable. Let us attempt to present briefly the materials on 

 determining the specificity of parasites, with the reservation that primarily 

 we shall utilize Russian works because this question is especially debated 

 in our native literature. The simplest definition of specificity is the defi- 

 nition of V. A. Dogiel, namely that the specificity of the parasites is a 

 "certain conditioning of determined species of parasites to determined 

 species of hosts, " with the reservation that specificity can vary a great 

 deal (Dogiel, 1947). However, at the present time this definition is con- 

 sidered insufficient and even erroneous by the majority of parasitologists, 

 and new definitions, much more complicated in comparison with the ones 

 cited above, have been offered (Markov, 1953). Thus, G. C. Markov (1953) 

 writes in his doctoral dissertation: "Specificity is the hereditarily con- 

 solitated, relative morphophysiological specialization of the parasite to 

 the host or a determined group of hosts, formed under the influence of the 

 conditions in the existence of the parasite, " adding to this that specificity 

 represents a specific characteristic of the species. S. S. Shulman (1954b) 

 defines specificity as "the historically formed, ecologically conditioned, 

 supported by natural selection and hereditarily consolidated adjustment of p. 284 

 the parasite to its host. " Finally A. V. Gussew (1955) gives the following 

 definition of specificity: "Morphophysiologically and ecologically conditioned, 

 historically formed- -as a rule under conditions of certain stabilization of 

 the medium, hereditarily consolidated and relatively stable adjustment of 



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