1813-1914.] 5 1 



species of fish in this area are so closely related that they are 

 manifestly the descendants of a common ancestor. Now where 

 did this common ancestor live ? A circle can be drawn round the 

 area inhabited by these Pollan-like fishes, all of which belong to 

 the genus Coregonus. The centre of distribution lies in the Irish 

 Sea, and all the localities where these species live drain into the 

 Irish Sea, except the Irish localities. It is not unlikely though 

 that all the Irish lakes referred to formerly emptied their waters 

 into the Irish Sea. At any rate there are reasons for the belief 

 that the west of Ireland formerly stood at a higher level relatively 

 to the east than it does now. If we assume that all the lakes 

 where these Pollan-like fish live once communicated directly with 

 the Irish Sea, we can easily imagine that the home of the ancestor 

 of these fishes must have been in the area occupied by the Irish 

 Sea. I have stated before that when the land stood at a higher 

 level the northern part of the Irish Sea was probably occupied by 

 a large fresh-water lake, and I presume that the ancestor of the 

 Pollan and its relatives inhabited that lake. Now, if the sea had 

 gradually encroached upon the land from the south, as 1 suggested, 

 it would finally have converted this large lake into a gulf or ocean 

 bay, with the result that the inhabitants would have taken refuge 

 in the rivers and lakes that drained into it. The fishes would have 

 become scattered east and west to escape the northward advance 

 of the sea. In their various new homes they would have acquired 

 the slight distinguishing characters which now separate the fishes 

 belonging to this Pollan group. A theory such as this appears to 

 me to account fairly well for the occurrence in the area indicated 

 of closely related forms. At the same time it tends to support 

 the conclusions I had arrived at from the general consideration of 

 the terrestrial fauna of Ireland. 



Still another branch of enquiry remains which I have not yet 

 touched upon in this brief survey. It is not necessary that we 

 should limit our studies to the creatures inhabiting the land or 

 fresh water. The marine fauna also throws light on the problems 



