IxXX CLASSIFICATION. 



occasioned by shocks or injuries when fission may ensue occasioning double- 

 headed monsters ; or crossing two distinct species would seem to set up 

 various- abnormal structural results. Analogous variation, or where an 

 animal varies towards some allied form, has not, I believe, been observed in 

 fish. There is also atavism, or the reversion towards the original type. 



In addition to zoological classification, fishes are likewise divisible in 

 accordance with their distribution and habits ; thus some may be termed sea 

 or marine forms, while others are fresh-water residents. Also there are 

 species which observe no such undeviating peculiarities, as they are observed 

 to roam from one of these localities to the other, especially such as are 

 generally found in brackish water. 



Marine forms are likewise subdivided in accordance with the areas 

 they principally frequent ; thus inhabitants of the open sea are known as 

 pelagic : such as enter fresh waters for breeding purposes as anadromous : 

 while the along-shore forms which are restricted within a tract up to 80 or 

 100 fathoms, rarely entering fresh waters or being found in mid-ocean, as 

 littoral.* 



Fresh-water forms are those which pass their entire lives in fresh waters ; 

 while such as live in fresh waters, but descend to the salt water to breed, are 

 termed catadromous. 



If we now attempt further divisions, commencing with marine forms, a 

 difficulty meets us so soon as we try to classify the various sea-fishes into 

 residents of certain zones of depth, the majority changing their feeding- 

 ground at different periods of their existence. They may also be influenced 

 directly by heat and cold, sunshine and darkness, storms and calms; or 

 indirectly by how these influences affect the products on which they feed. 

 Thus occasionally pelagic forms are observed in vast numbers at the surface, 

 where as a rule they are generally absent. The young also of many pelagic 

 forms pass their infantile life in the littoral zone, but when adult retire into 

 deeper waters. The Antennarius is found in some portions of the globe, 

 living near the surface in its early age, revelling in the pleasures of light and 

 warmth ; but as time creeps on, some have been observed to sink to the 

 bottom, where they can indulge at their ease in a sluggish existence, if it can 

 be a pleasure to them to live where seasons are absent, aud day and night 

 unknown. The deep sea forms of animal life are observed to be mainly 

 composed of more or less modern shallow-water genera, and their allies, which 

 have extended their range into the deep sea zone. 



In spite of the foregoing difficulties further subdivisions of fish have been 

 instituted, thus in marine forms we have such as inhabit regions through 



* A. Agassiz considers the littoral zone as up to 100 or 150 fathoms, or an extension of the coast- 

 line to a depth at which the direct action of the sun's rays is limited : that from it in a slope up to 

 450 or 500 fathoms is a continental zone where the diminution of temperature is rapid ; beyond this 

 is the abyssal, where there is a low temperature varying little from freezing-point. 



