Remarks on Myctophum glaciale (Reinh.). 39 
and spring, but to a certain extent also the whole year round. 
Relict forms of northern origin often have their season of pro- 
pagation transferred to the colder months of the year in their 
soutkern area of distribution. The spawning in winter of the 
Mediterranean race may be explained in this way, indicating the 
northern origin of the species. That the spawning of the race 
thori is not restricted to a certain period of the year but partly 
comes off the whole year round must, I think, be considered as 
a secondary developed feature caused by the more uniform hydro- 
graphical conditions of this region throughout the year." 
There is a question which naturally arises, when the view 
here advocated is accepted, namely why did not M. glaciale adapt 
itself to the new conditions outside Gibraltar as well as inside 
of it. The hydrographical conditions of this area of the Atlantic 
seem to come nearer to those of the northern Atlantic than do 
those of the Mediterranean. This is particularly so with regard 
to salinity which is extraordinarily high in the Mediterranean. 
The temperatures of the upper 500 metres are much the same 
on the Mediterranean side as on the Atlantic side of Gibraltar 
(16—13° C. in summer), but farther out in the Atlantic the tem- 
peratures of this zone are somewhat higher (20—14° C.), at 
least in the vicinity of the Azores. There is, however, another 
circumstance which may have played a more important role in 
the isolation of the Mediterranean stock. I shall not trench on 
the much-debated problems connected with the Glacial Epoch, 
the causes of the origin and the decline of this period, but only 
mention this. The termination of the Glacial period must, I think, 
have been preceded by hydrographical changes of the waters 
off western Europe (why these changes took place need not be 
discussed here) and the new conditions established sooner there 
than on the European continent. As an active pelagic species of 
boreo-Atlantic character M. glaciale receded northwards and the 
Atlantic stock was cut off from the Mediterranean one. The 
hydrographical conditions of the Mediterranean changed also but 
*) The time of spawning does not seem to be the same in the different 
regions of the Mediterranean. The remarks above refer to the main centre 
of distribution, viz. the western basin. Acc. to Tåning (1918 p. 45) the 
Marmora specimens seem to divide more sharply into two size-groups than 
the Mediterranean ones, which would indicate a well-defined spawning time. 
