53 



the conditions which is measured by the distribution. The migra- 

 tions are altogether denatant, and being passive cannot be said 

 to depend upon the physical conditions within the area in which 

 they take place. 



It follows that if any of the marine forms are subject to other 

 circumstances than currents in their migrations it must only be 

 those which have the power of contranation, viz., the majority 

 of the fish, some of the Crustacea, and the larger at least of the 

 Cephalopod MoUusca. 



Even with regard to some of these it is at once evident that 

 like the plankton their migrations are conditioned only by the 

 circumstances of the area of distribution. If we admit Schmidt's 

 claim that there is only one spaw^ning ground for the eel, we have 

 one school, a larva of w^hich may be drifted to any place between 

 Greenland and Morocco. The migrations therefore are not 

 materially departed from under conditions which vary from 

 arctic to tropical. They depend entirely upon the currents of 

 the North Atlantic. The same is true of the many fishes which 

 in the egg and larval state are drifted from the ocean to the 

 continental shelf and shore. 



The question is thus narrowed to a consideration of the species 

 of the banks and shallow water which are divided into a series 

 of schools. So far as distribution is concerned many of these are 

 as widely spread as those which have been .considered. The 

 species then are resistant to conditions which may frequently 

 vary from arctic to tropical. It would be difficult to specify 

 the conditions of temperature, salinity and food in the case of 

 such species which regulate the migrations generally, and when 

 we remember that the schools are intimately related w^e should 

 hesitate even before we conclude that each is modified in its 

 migrations by the differences which could be indicated for the 

 regions where they occur. It is a question also, it must be said, 

 which must not be confounded with that of the relative fitness 

 of the environment in the several parts of the area of distribution, 

 for this only concerns the density of the population, not the 

 migrations. 



Because the plaice of the Forth and the Northumberland 

 schools migrate north, as it is plain those of the east coast of 

 Scotland do as a whole, and are therefore different from the plaice 



