58 The hish Nahiralist, March, 



territories indicated, but by zoologists who are examining the 

 distributional phenomena. Two of them, for instance, Mysis 

 relida and Pontopojxia affinis, have also been found in Lakes 

 Superior and Michigan. The former, which I have taken in 

 lyOUgh Neagh and Lough Krne, which is its most southern 

 station, occurs in ten Scandinavian lakes, several in Finland 

 and Russia, and a few in North Germany and Schleswig. Now, 

 if so many ages ago a marine animal was isolated from its 

 marine surrotmdings, and introduced separately in isolated 

 inland freshwater basins differing from each other in depth 

 and extent, climatic conditions, and probably food supply, 

 how comes it that the various vicissitudes have not developed 

 in each case some adaptive changes in the organism ? But we 

 find that M. relida is very uniform in its features, and only in 

 a few cases offers trivial differences over so wide an area of 

 distribution. If we consider the vast size of Lakes Ontario and 

 Michigan, whose waters stretch for hundreds of miles, Lake 

 Onega, also, of about 150 miles in length, and of great pro- 

 fundity, their winter temperature, too, and those of the Finnish 

 Lakes, and compare these conditions with those of our lakes 

 of Erne and Neagh, whose extreme soundings are 200 feet and 

 100 feet respectively, but with the greater areas of both com- 

 paratively shallow, and their temperate climate, the zoologist 

 is almost forced to conclude that this freshwater species has 

 been accustomed to this medium for ages previous to the 

 Glacial epoch. The modification from the marine animal 

 when introduced to fresh water would, of course, be due to a 

 specific form of environment ; but apart from the considera- 

 tion that nature seems always to find more than one way out 

 of any difiiculty, the changes in this case would be complicated 

 by the local peculiarities above adverted to. And with regard 

 to the testimony of geology, authorities seem unanimous in 

 stating that insurmountable difficulties oppose any theory of 

 post-Glacial submergence, which would have admitted the sea 

 into the chain of American lakes above the Falls of Niagara. 

 The lower portion of Lake Ontario admittedly has a deposit 

 containing sea shells of comparatively recent geological date, 

 but only the lower portion. How then did Mysis rclicta become 

 introduced into our two great Irish lakes ? They present 

 little difficulties in regard to the submergence theory, but is 



