MARINE ANCESTRY. 5 



and this intolerance acts as a barrier against tlieir diffusion. Consequently if tlie 

 inland waters of oceanic islands are examined true fresh-water forms are absent, 

 unless at some former period a land connection had existed with a continent or 

 these fishes had obtained access accidentally or been imported by man. But there 

 are certain forms of fish life in our fresh waters which are evidently descended 

 from a marine ancestry as eels or perches, and these are much more tolerant of 

 salt water than are those which are of purely fresh-water descent. * 



But while it is difficult to show fresh-water forms that have changed to 

 a marine residence, it is very easy to find sea fishes as temporarily or permanently 

 residing in fresh waters. Malmgren drew attention to certain marine species as 

 the four-horned bullhead. Coitus qiiadricornis, a sucker, Liparis barbatus, and a 

 variety of the common herring, Glupea harengus, being present in the northern 

 portion of the Baltic, where the sea is now least saline, whereas they seem to be 

 absent from its southern extremity where they might well be looked for had they 

 obtained an entrance fi'om the North Sea. These forms are smaller when residing 

 in the Baltic than are those of the same species living in the Arctic Ocean, and it 

 has been reasonably concluded that they are the remnants of a fauna which at one 

 period was common to both localities. During the latter portion of the glacial 

 epoch most of Finland and the middle of Sweden were submerged, so the Baltic 

 must have been a gulf of the glacial ocean, for the entrance to the south at the 

 Cattegat had not then been opened. As the Scandinavian continent became 

 elevated, the Baltic became cut off by this raised land from the Ai-ctic Ocean to 

 the north, but it opened to the south through the Cattegat into the North Sea, 

 and its fish-fauna even now retains representatives of its former marine northern 

 glacial fauna, as well as such immigrants as have arrived through the Sound.* 



The foregoing must be classed among instances in which marine forms have 

 inadvertently become imprisoned in water which has steadily changed its 

 character from true saline to that which is only a little more than brackish ; here 

 some live and breed, one can scarcely add thrive. But we shall presently have to 

 allude to the land-locked salmon of Lake Wenern which demonstrates how an 

 anadromous form may be similarly impi'isoned in fresh water and still flourish. f 

 Even British waters possess, besides members of the salmon family, anadromous 

 forms, or such as ascend our rivers for the purpose of breeding, as shad, 

 Glupea alosa and C. jlnta, also flounders, Pleuronectcs flesus, while many other 

 mai'ine fish, as bass, gray mullet, turbot, soles, plaice, and smelts have been 

 naturalized in fresh water where some have continued their kind. 



When we find that some members of a genus inhabit the sea and others are 

 restricted to the fresh waters, as among the Coregoni, we are probably not far 

 wrong should we conclude that their ancestry was marine. The houting, 

 Goregonus oxyrhy)iclius, is found along some of the European coasts, and it ascends 

 into fresh water, while Mr. Ogilby has observed the pollan, G. pollau, in Ireland 

 descending to the sea. Respecting the salmon, trout, and char, the most diverse 

 opinions have been and are still held as to whether their ancesti'y was marine 

 or fresh water.J 



* Inherited instinct appears to induce those Arctic forms to seek a passage to the north : while, 

 due to the alterations in the physical character of the water in the Baltic, which is continuously 

 decreasing in salinity, they are a smaller and more miserable race than are their relatives which 

 still reside and thrive in the Arctic (Jcean. 



t In tropical comrtries, as India, it is not uncommon to find marine fish detained in pieces of 

 fresh water, to which they obtained access under varying circumstances. Some forms of fishes enter 

 inundated grounds during high sjjring tides, or are carried over banks, and often find it impossible 

 to return to the sea ; here they Uve until the succeeding rains cause floods and allow them to escape. 

 The probabilities are that species of gar-pikes as IJelune cancila, some herrings as Eiigraulis 

 teliira, the pretty globe fishes, Tetrodoncutcutia, and numerous other forms now entirely confined 

 to fresh water in India, must have descended from a marine ancestry, for representatives 

 of the same genera are numerous in the contiguous seas. 



{ Pennant (Brilish Znoloijy, iii, 1776, p. 288) observed that " the salmon is a fish that lives 

 both in the salt and fresh waters, quitting the sea at certain seasons for the sake of depositing its 

 spawn in security." Fleming (1828, p. 179) considered the salmon as a "migratory fish from 

 the sea." Parnell (1838, p. 279) remarked " there is no doubt that the true abode of the 

 salmon is in the sea, for as soon as it has entered the rivers it begins to deteriorate in condition, 



