Ixxxvi GEOGEAPHICAL DISTRIBUTION 



prey, while others, as the Antennarius, drift about on sticks or seaweed iu' 

 accordance with the action of winds and currents. Among them are some 

 which ascend to the surface during the night-time, and may be possessed of 

 luminous organs (page xxv}, common to them, and likewise to some o£ the 

 deeper abyssal species. 



Littwal forms (see page Ixxx), although constantly migrating within the 

 limits of their own zone, or even extending their range to within the localities 

 frequented by pelagic species, will often decline to pass deep-sea ravines to 

 dpposite banks, or cross over ledges of rock._ 



GEOGRAPHICAL DISTRIBUTION OF BRITISH FORMS. 



Among the nineteen genera of fresh-water fishes. which inhabit the British 

 Isles, the following are, (I) common to the Palsearctic and Nearctic regions : 

 1. Gasterosteus, 2. Perca, 3. Gottus, 4. Lota, b. ,8almg, 6. Thymallus, 

 7. Goregonus, 8. Hsox, 9. Leuciscus, 10. Ahramis, 11. Sturio; (II) forms 

 restricted to the Paleearctic region : 12. Acerina, 13. Gobio, 14. Tinea, 

 15. Ahramis, 16. Alburniis ; (III) genera present in the Palsearctic and 

 Oriental regions : 17. Gyprinus, 18. Carassius, Id . Nemacheilus ; (IV) found 

 in the Palsearctic, Oriental, and Ethiopian regions : 20. Barbux. 



Among the foregoing, eleven, or more than half of the British fresh- water 

 genera of fishes, are common to both Arctic regions, while those numbered 1, 

 3, 5, and 7 have marine representatives. Nos. 2 and 11 are frequently found 

 in salt water, the former b^eing closely connected with the marine perches, 

 .and Lota, possibly the remnant of a glacial ocean, is closely related to marine 

 forms ; but Leuciscus, Ahramis, and Esox are distinctly restricted to fresh 

 waters, yet are found in both regions, whereas the sea as at present existing 

 would form an insuperable barrier against their normal extension from one 

 point to the other. It has been advanced that all evidence points to a 

 continued mild climate in the Arctic regions through Cretaceous, Eocene, 

 and Miocene times, whereas had the North Atlantic between Europe and 

 North America been closed, although such might have raised the temperature 

 of these isles, it must have increased the cold in the Arctic regions by cutting 

 off the gulf stream. Appearances, as regards the distribution of mammals, 

 seem to point to there having been probably on more than orie occasion, but 

 for brief periods during the tertiary period, a land connection between 

 N.W. Europe and N.E. America, and to this the distribution of the strictly 

 fresh-water genera of fish would seem to lend countenance. 



Gyprinus and Garassius appear to have been forms introduced into our 

 isles, while the little loach Nemacheilus is found in a continuous chain of 

 many species throughout the Palsearctic and Oriental regions. 



