﻿That one form« thus in the course of a few generations can change 

 into another »form« is not without known parallels in the animal and vege- 

 table kingdoms, and it is evidently a very common phenomenon in nature, 

 though one, I am sorry to say, which has hitherto been but little regarded. 

 — I shall here only remind the reader of certain species of Crustacea (Ar- 

 temia) which, when carried from fresh into more saline water, can change 

 from one species to another in the course of a few generations, nay, even 

 to another genus, and of certain species of ferns (of the genus Adiantnm) 

 which, in the course of six generations, when cultivated on a soil containing 

 serpentine, will change into other species, which not only in form hut also 

 in various biological relations (green or not green in winter) are different 

 from the former. — 



Resume: It is not possible then, even within a certain sea, to indicate 

 a certain length which distinguishes all grown-up and mature plaice from all 

 not grown-up and immature; for all plaice do no become grown-up at the 

 same length, even irrespective of the difference which is owing to the sex; 

 but it is possible to indicate an approximate length which in broad features 

 distinguishes the mature fish from the immature i the fry. 



This length is different in the different seas: the Limfjord together 

 with the German Sea, the northern and southern parts of the Oattegat, the 

 Belts and the Baltic east of the isles of Sealand and Falster. 



Having shown in the above section that the mature plaice have a 

 different size in our seas, I shall here give some particulars with respect 

 to the occurrence of the immature fish, the fry, and show that the latter 

 also in open nature are arranged in groups according to their size. I shall 

 show how many of these groups we may count, a matter which will eluci- 

 date various things respecting the growth of the plaice, and especially give 

 information as to the question of how many years it requires before it 

 becomes mature, 



In order to find out whether a number of such groups arranged ac- 

 cording to size ( annual series«) may be proved to exist of plaice in a 

 given sea at a given time, as it has been proved with respect to other 

 Hat-fishes, the northern Cattegat (at the Skaw, Aalbsek, Jerup, Frederiks- 

 havn, Bangsbo, Sseby, Lseso Rende, off the Mariager Fjord) was particularly 

 explored, several times in 1891, table VI, column 1 — 2, in 1892, table VI, 

 column 3—17, in 1893, table III & IV. In 1891 -92 the explorations were 

 carried on by myself onboard the »Hauch< or »Hav0rnen«, in 1893, 



