22 



and tlieir flowers are of no use for the propagation. Why must then al 

 eggs of plaice be shed under favourable conditious? After all, we have al- 

 ways thought that only a very small number of them, on the whole, become 

 growu-up fish, even under the most favourable circumstances. To my thiuk- 

 ing, there is notiiing to prevent us from accepting the facts we obtaiu 

 by our investigations in nature, even though they go greatly against 

 what we usually call "expediency". If the eggs of the plaice cannot be 

 developed in one sea, then the stock will die out, by little and httle, if 

 not an immigration takes place from somewhere else. If it does uot take 

 place the stock is lost; this, in my opinion, is all the expediency that is 

 found in nature. 



It is not only in the Baltic Sea, however, that the fry of the plaice 

 belonging to the first bottom-stages has auother geographical distribution 

 than the growu up fish. I dåre say, this is the case in nearly all piaces 

 where the plaice lives, for iustance in the North-Sea. Here the fry is 

 found only ou a quite narrow strip along the shore, ou low water, while 

 the central parts of the North-Sea are peopled by the older fish only. 

 Consequently, the plaice cannot live its whole life out here either, and only 

 the eggs and yonug fish which come near the shore at the right time ean 

 be developed. We do not know how great a part of the whole number 

 of eggs and pelagic young fish is condemned to deatli from the beginniug, 

 because they have got into unfavourable curreuts which do not carry them 

 landwards; but we have no right to doubt that many are destroyed before 

 they are developed, already ou account of the hydrographical couditions. 

 If this is the state of things in the particular home of the plaice, the 

 North-Sea, why should it not be highly probable that still greater diffi- 

 culties arise, which may prevent the developmeut of the eggs, where the fish 

 is near the limits of its distribution, for instance in the Baltic Sea, where, 

 moreover, the conditions as to salinity make matters very complicated indeed. 

 There must be something which attaches the plaice lo the area within 

 which they are actually distributed, or ratlier, there must be something 

 which prevents the plaice from pas.sing the limits of this area; otherwise 

 such limits would not exist. Nothing seems to me more probable then, than 

 that it is the grown-up fish, especially, which eau best try to pass these 

 limits and which actually also do so, but that the tender fry is stopped 

 by the ditficulties, for instance, the differeuces in the physical conditions. 

 I do not see, therefore, that there is anything" to prevent us from supposing, 

 that the full-grown aud half-grown plaice very well can live in the Baltic 

 Sea, and that the eggs can be shed there, but that the latter cannot go 

 through their whole developmeut every where, where they are shed, wheu 

 the results of the investigations, in ]ioiut of faet, lead to this supposition. 

 To speak of "die innere Uuwahrscheinlichkeit von dieser Annahme" seems 

 to me to be quite groundless. 



That the fry and the growu-up fish of many species have uot always 



