52 HIGH SCHOOL ZOOLOGY. 



generally considered a sufficient ground for regai-ding them 

 as distinct species. Varieties are often the result of local 

 conditions and are tlierefoi-e spoken of as geographical varie- 

 ties or sub-species, but they may be also brought about artifi- 

 cially by man, in which case they are generally spoken of as 

 "races." 



2. Certain groups of species resemble each other so much 

 that they are grouped by naturalists under the same "genus." 

 Some genera are large, embracing a number of species, others 

 small with only one or two; when they are large it is con- 

 venient to arrange the species in smaller groups, which may be 

 designated by sub-generic names. In this way instead of the 

 binomial system, a polynomial system may be adopted in which 

 the name of an animal may have four parts, the generic, sub-ge- 

 neric, specific, and sub-specific names. Although too cumbrous 

 for general adoption this system has the merit of requiring a 

 close attention to variation, which is one of the most inte- 

 resting questions in Natural History. 



3. In regard to the species we have been studying, the 

 generic name Amiurus embx-aces a large number of different 

 kinds of catfish from different parts of North America. Of 

 these different kinds three occur within our region, viz.: 

 A. nebulosus, A. natalis (Le Sueur) Jordan, (the yellow catfish), 

 and A. vulgaris (Thompson) Nelson, (the long-jawed catfish)* 

 some six other species are more southeily forms. The genus 

 is a " difficult " one, the species being hard to characterise and 

 it is doubtful whether all the species are "good." For example 

 there is a southern form which is sometimes regarded as a dis- 

 tinct species \A. marmoratus (Holbrook) Jordan], sometimes 

 mei-ely as a mottled variety of our northern form, and conse- 

 quently named by the zoologists who hold this view A. nebulosus 

 var. marmoratus. 



