21 



Some General Remarks on the Distribution of Various Species 

 of Fish, as Fry and as Grown-up Fish. 



Tlial the eel passes certain ])eriods of its lit'e in fresii wtiter, or in sliel- 

 tered seas in salt water, without breeding there, for instance, in the whole 

 of the Baltic Sea, and therefore naust undertalce long migrations to go 

 through the remaining part of its life; that it has, in short, a quite differ- 

 ent geographical distribution in the different periods of its life, is now 

 grauted, I thiuk, by most people. That somethiug like this holds good 

 also of mauy otiier fishes, lias long been known; the salmon breeds in 

 fresh water, and lives at other times in the sea far from its native ])lace. 

 These migrations have been termed breeding- migrations, and tiiey have been 

 compared to those of the migratory birds which we all know. By little 

 and little the faet of these migrations has become generally accepted. But 

 ■\ve have had great difficulty in reconciling ourselves to the tliought that a 

 Rsh like the plaice should not go through its whole developmeut everywhere, 

 wherc tlie grown-np plaice are found. And yet we know that the floitudcr 

 ofteu lives in fresh water and gets ripe eggs, although it cannot breed there 

 so that (tnii fry iviU come out of the eggs. Now, it is no doubt the rule that 

 the flounder tries to emigrate to the sea in order to shed; but it is often 

 prevented from so doing by natural hindrances, artificial dams, or such like, 

 which the small fry from the sea may be able to penetrate perhaps, but 

 which are impassable to the grown-up fish. We have here then, close at 

 hånd, an example which reminds us of the plaice in the Baltic Sea. And 

 how can we expect, after all, that animals requiring such different conditions 

 of life, as the young plaice ou warm sunshiny .shores and the grown-up fish 

 which prefer darkness and cold water on the deep, should have the same 

 geographical distribution? Is it strange that the fry stops somewhat earlier 

 on the long and winding way from the Gattegat into the Baltic Sea, than 

 the grown-up fish? Evidently, however, it is not this either which has 

 appeared so inapprehensible to many people; rather the faet that the lylaice 

 of the Bultic Sea does not emigrate every year to shed its spanm at more 

 favourable piaces, i. e. piaces where the eggs can be developed into fry. 

 Such an inexpediency seeras to many inapprehensible. To be sure, if all 

 plaice were as unfortunately situated, it would, evidently, be inappreheu.sible, 

 if not the eggs, after all, were carried away by the curreut to more fa- 

 vourable piaces, but of this last, however, we know as yet nothiug positive. I 

 believe this way of thinking is a remnant of old learning which was good 

 philosophy perhaps in its own time; but if we want to understand tlie 

 natural conditions as they really are, we had better guard against such 

 preconceived opinions. Botanists, for instance, know several examples of a 

 huge wasting of propagative products in nature. Several Phanerogams 

 are distributed in such a way that through large regions one sex is found 

 only, the other not; the piants are then reduced to vegetative reproduction, 



