432 MR. K. GUKNBY ON THE CKtJSTACEAN PLANKTON 
the lakes themselves are glacially-eroded rock-basins is not universally 
admitted. In any case, however, the fauna must have reached these lakes 
after the withdrawal of the ice. 
According to Clifton Ward (1876) the passing of the Glacial period was 
immediately followed by a submergence of the whole district by as much as 
2000 feet, which would' leave only the highest ground i)rojecting as a 
number of small islands in the midst of an arctic sea. If a submergence to 
this or to a much smaller extent can be admitted, then L. macrurus might 
well be a relict of this sea left in Ennerdale. Unfortunately Ward's 
conclusions are not admitted by geologists of the present day, though I have 
not been able to find in any literature available to me any serious discussion 
of the question of post-Glacial changes of level in this district. The matter 
is hardly referred to in Marr's ' Geology of the Lake District,' but 
apparently he does not admit a submergence of more than a few feet, which 
could not have made any material difference to the distribution of land and sea. 
If a submergence to an extent sufficient to bring the sea in the first 
instance into direct connection with the Ennerdale basin cannot be conceded, 
there are only two alternatives. Either the interpretation of the geological 
facts is wrong, or a marine species has either been able to migrate for miles 
up a rushing stream of fresh water or has been transported directly into the 
lake. No biologist would for a moment consider such migration or trans- 
ference to be even conceivable. It must be remembered that we are dealing 
with a species which occurs in this one lake only in the British Isles and 
cannot have been transported to its present habitat from any other lake in 
Britain. 
■ Even if Clifton Ward's great submergence could be proved true, it is still 
a little doubtful if it would satisfactorily explain the facts. Our knowledge 
of marine relicts seems to point to the conclusion that transformation of 
marine species into fresh-water relicts has rarelj' happened, and only in cases 
where very large areas of water have been involved. Ilius the L. macrurus 
of the Baltic area owe their origin to the transformation of the Yoldia Sea 
into the Ancylus Lake, rather than to the separation of lakes from (he Yoldia 
Sea itself. In other words, L. grhnaldii was transformed into a form of 
L. macrurus in the Ancylus Lake and not, in the first instance, in the 
existing inland lakes. Similarly, we should not expect that a race of 
L. grimaldii could have survived in so small a lake as Ennerdale itself even 
if it could be shown that the lake had itself been cut off from the sea. 
To explain the occurrence of this species in Ennerdale in the same waj' as 
its occurrence in the Baltic lakes, it is more reasonable to assume that the 
Irish Sea itself has passed through a history somewhat similar to that of the 
Baltic, having been changed from an arctic sea containing L. grimaldii to a 
fresh or brackish water, and finally to its present condition, and that during 
the second phase L. grimaldii became in part transformed in the direction of 
L. macrurus. Further, that conditions were such that, on the renewed 
