272 FA, BATHER [ocTroBER 
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;* 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 (Ostrea, 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 
Bohustiin, 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 ina fjord. In both cases is a narrow, enclosed water which com- 
1 Occupying more or less the district of the present Baltic. 
