lxxxiv DIFFUSION- AND DISTRIBUTION. 



in Burma, and a species of saw-fish, Pristis Perrotteti, upwards of four feet 

 long, in the Mahanuddi river in Orissa, in fresh water and at least forty 

 miles from the sea, M. de La Gironniere found a saw-fish inhabiting fresh 

 waters in the Laguna de Baij, Luzon, and which was identical with those in 

 the Gulf of Manilla. Gill has observed upon a saw-fish, and a shark being 

 taken in Lake Nicaragua. Advantage has been taken of the knowledge of how 

 marine forms 'will live in fresh water, and the Romans turned it to a practical 

 account, by forming fresh-water vivaria wherein some marine fishes were 

 kept. Mr. Arnold has been similarly successful in Guernsey (vol. i, page 229), 

 and Colonel Meynell in Yorkshire (vol. ii, page 123). 



The intolerance of fresh-water fishes to sea-water* is greater than of 

 marine ones to fresh. Such forms as perehes or sticklebacks, whose ancestors 

 were probably marine, are more .tolerant of saline water than the more strictly 

 fresh-water species as carps. ' This question, of course, is most important 

 as regards the dispersion of these forms, - for should they not be able to live 

 in anything but fresh water, for them to extend from one point to another 

 practically requires a land connection. Lubbock tells ' us that on sea-floods 

 occurring in the Norfolk Broads, the first fish to suffer are the tench, pike, 

 bream, and roach ; perch bear a strong admixture, but catadromous eels are 

 unaffected. If, as is ' the case in Oceanic Islands, as the Andamans of the 

 Indian seas, we 'find fresh- water fishes similar to those on the mainland and. 

 at Ceylon, it seems reasonable to suppose that although the two localities 

 may be hundreds of miles distant, a land connection must have existed 

 between them at some antecedent period of the world's history. If volcanic 

 islands are searched true fresh-water fishes are absent,- unless they have 

 obtained access by accident, or been placed there by man. Respecting the 

 northern limits of fresh-water fishes it has been ascertained that they are 

 absent from regions where ice is nearly or quite continuous. 



Abyssal forms (see page lxxxi) are found in the deep abysses of the ocean, 

 where there is an entire absence of light obtained from the surface, .but 

 whether there are not present means for the production of luminosity I have 

 already touched upon (page xxv). Deep-sea fishes have either very large 

 eyes, similar to nocturnal land animals, or else they are quite blind, and as 

 regards these organs they show a considerable resemblance to what is 

 observed in such as inhabit waters in dark caverns. Recent investigations 

 would seem to show that from eighty or a hundred fathoms in depth up to 

 two hundred fathoms the size of this organ increases, in order to collect any 

 rays of light, but beyond this last depth both large and small eyes are found ; 

 the latter forms have also usually tentacles, for the purpose of feeling, 



* As bearing on this point, marine Medusa are very intolerant of fresh water, hut a fresh-water 

 form having been discovered, it was found to be even more intolerant of sea-water than the marine 

 ones are of fresh (Nature, June 24th, 1880). 



