272 F. A. BATHER [october 



Southern forms, on the contrary, enjoy a comparatively equable and 

 high temperature and constantly Salter water. Now, the water of the 

 Sound is at all times of comparatively low salinity, and is, under the 

 influence of winds and currents, liable to still greater reduction. More- 

 over, its shallowness, the influx of cold Baltic water, and the cold 

 winds blowing from Sweden, combine to lower the temperature in 

 winter almost to freezing-point to great depths, if not to the very 

 bottom of the whole Sound. These conditions thus, while suited to 

 the hardy northern species, are distinctly unfavourable to the more 

 southern forms with which they contest the ground. 



It is then intelligible that Arctic forms should continue to live in 

 the Sound ; but, since they have not entered recently and are not 

 now coming in, they must have persisted there or thereabouts since a 

 time when Arctic conditions were so widely extended that they em- 

 braced the now isolated Sound as well as the intervening areas. 

 That took place during late glacial times. During the changes that 

 succeeded, these Arctic forms must have changed their home and given 

 way before the fresh-water streams from the Ancylus-sea, ; 1 but though 

 many doubtless perished, a number of forms could brave it out, thanks 

 to their power of resisting brackish water. When a fresh sinking of the 

 bottom of the Sound let the salt water burst afresh into the Baltic, the 

 Arctic forms came along with it by degrees, into the Sound and the 

 Belt, and perhaps yet further ; in this way they withdrew from the 

 contest with the more southern forms that were now thronging up out 

 in the Kattegat. This struggle with the more southern and more 

 typically marine forms was then for a time even harder than now, since 

 for a long period the water was much Salter than at present, so that 

 the oyster, Tapes, and other forms now extinct in those parts, could 

 thrive there. It is therefore probable that it was just at that time — 

 the Littorina period — that the break took place in the connection 

 between the northern and principal area of distribution of the Arctic 

 forms, and the more southern isolated districts, such as the Sound, 

 where those forms still exist. After a time the Kattegat again became 

 less salt, and a part of the southern marine forms (Ostrca, Tapes, etc.) 

 died out. Thus began the existing state of things, in which the Arctic 

 forms again found favourable conditions of existence, and possibly again 

 extended their range. 



Thus it is that, in the existence of an Arctic element, the fauna of 

 the Sound presents a phenomenon like to that of Gullmarsfjord in 

 Bohustan, and many Norwegian fjords, in which Arctic animals are 

 found far south of their proper limit. Such persistent types are called 

 relicts ; and thus the fauna of the Sound may to a certain extent be 

 called a relict fauna. The same term is perhaps also applicable to the 

 fauna of the Belt. The conditions in these sounds are in a way like 

 those in a fjord. In both cases is a narrow, enclosed water which com- 



1 Occupying more or less the district of the present Baltic. 



