INTRODUCTORY 29 



South of England, became covered with ice; then a submergence of 

 the land perhaps brought the sea right to the edge of the ice-sheets, so 

 that any true fresh-water fishes which may have been in our rivers 

 would have perished. At the end of this cold period, a gradual 

 elevation of the land took place, culminating in the union of our islands 

 with each other and with the Continent, and then a subsidence followed 

 which gave to our islands approximately their present outline. 



" Speculations as to the exact time occupied by these changes 

 are futile, but it is probable that the whole duration of the Tertiary 

 Period should be expressed in tens of millions of years and that of the 

 Glacial Epoch in tens, or at most hundreds of thousands. Possibly 

 about 100,000 years may have elapsed since the end of the Glacial 

 Epoch, and our final separation from the Continent was of still more 

 recent date. 



"As soon as the ice-sheet had begun to disappear char must have 

 commenced running up into the lakes which were formed, and as the 

 elevation of the land proceeded new lakes appeared and in turn became 

 inhabited by Char. All this time the climate was gradually getting 

 warmer, and the southern limit of anadromous Char was receding 

 northward; then the sea-trout reached our islands, and these in turn 

 began to form fluviatile colonies in our lakes and rivers." 



It is clear from the foregoing that Mr. Regan asks us to believe, 

 and to me at least his reasoning is as plausible as it is interesting, that 

 all the trout which to-day throng our waters, trout great and small, 

 migratory and non-migratory, are the immediate lineal descendants of 

 certain enterprising trout which came up out of the prehistoric Atlantic 

 Ocean into our rivers to spawn when the great ice-sheet began to recede 

 before more genial climatic conditions. It seems to me, however, 

 relevant to observe that although Mr. Regan thus reasonably credits 

 our British trout with a marine origin, he has not thereby settled the 

 question of the primary and original environment of the invaders, and 



