lxxxvi GEOGRAPHICAL DISTRIBUTION 



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

 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 of the 

 deeper abyssal species. 



Littoral forms (see page lxxx), 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 

 opposite 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 Palaearctic and Nearctic regions : 

 1. Gasterosteus, 2. Perca, 3. Gottus, 4. Lota, 5. Salmo, 6. Thymallus, 

 7. Coregonus, 8. Esox, 9. Leuciscus, 10. Abramis, 11. Sturio; (II) forms 

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

 15. Abramis, 16. Alburnus ; (III) genera present in the Palaearctic and 

 Oriental regions : 17. Gyprinus, 18. Gar as silts-, 19. Nemacheilus ; (IV) found 

 in the Palaearctic, Oriental, and Ethiopian regions : 20. Barbus. 



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 being closely connected with the marine perches, 



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



forms ; but Leuciscus, Abramis, 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 one 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 Palaearctic and Oriental regions. 



