298 VAUIATIONS AND RELATIONSHIP OF 



north coast of Xorth America, and on the north coasts of Asia and 

 Europe. 



Attention may now be drawn to the hypothesis concerning the origin 

 of these closely allied forms, which the author tentatively puts forward. 

 It is pointed out in the first place that the Baltic may be considered 

 intermediate between the North Sea and the Arctic Ocean, as regards 

 the populations it contains. Further, the Baltic plaice, which differ from 

 those of the North Sea in the smaller number of their vertebra3 and fin 

 rays, and in the stronger and more conspicuous ctenoid scales which 

 they possess (characters which may be said to be masculine), would, if 

 these differences were intensified, come to resemble very closely the 

 Arctic form PL glacialis. On the other hand, the flounder in the 

 eastern part of the Baltic tends to vary in the direction of a greater 

 number of vertebr;\i and fin rays, and these characters are feminine. 

 PI. fjlacicdis may, then, be regarded as representing an extreme form of 

 either the plaice with masculine characteristics, or the flounder whose 

 characteristics tend to be feminine. Our author considers that of 

 the forms under consideration, the ^^/^aVe, judging from the rudi- 

 mentary development of its scales, is the oldest; that this form was 

 originally confined to northern latitudes ; and that it wandered thence 

 to the North Sea — without undergoing very much change ; and to the 

 northern coasts of Europe and Asia, where it gave rise to the variety 

 glacialis. At the same time an immigration took place into the Baltic, 

 through the Gulf of Bothnia, which was then open to the North, and 

 the plaice approximated to the glacialis type. As a result of the 

 influence of a mild climate, PI. glacialis became transformed to a 

 flounder-like form. This latter spread over the North Sea and the 

 Atlantic coasts of North America, and gave rise to a number of 

 varieties. 



In the meantime the original PI. glacialis of the Baltic disappeared, 

 and this form only now remains in the more northern latitudes where it 

 took its rise. It will be seen that this hypothesis attempts to account 

 for the differences which obtain between the North Sea and Baltic 

 plaice, and derives the flounder from a form like the plaice — the modern 

 PI. glacialis being regarded as representing an intermediate stage in the 

 evolution of the flounder. 



In conclusion, we may note that the author carefully refrains from 

 discussing the causes which have given rise to the local varieties he 

 describes. He does not even enter into the question whether the local 

 dil'ferences arise in the ontogeny of each individual, as the result of the 

 direct action of the environment, or whether they are inherited : in 

 other words, he does not, so far as we can make out, commit himself as 

 to the real nature of the local varieties — whether they are to be regarded 



