THE FISHES, MOLLUSOA, AND OTHER OBJECTS. 31 



Stickleback ((?. leiurus), in which the males exhibit 

 similar variations of colour, generally crimson 

 and purple ; and the Ten-spined Stickleback (6r. 

 pugnitius\ which also lives alike in fresh water and 

 salt, migrating up our rivers in shoals in the spring. 

 All the sticklebacks are most voracious, as every boy 

 who has angled for them is well aware. 



Space does not allow us to do more than point out 

 the evidence our fresh-water fishes, shells, and aquatic 

 plants afford of the very recent separation of Great 

 Britain from the main-land of Europe. And what 

 they indicate is abundantly verified by the strata of the 

 most recent deposits. We have no aquatic plants, or 

 mollusca, and few kinds of fresh-water fish, that arc 

 not common to continental rivers, lakes, and ponds. 

 And our English stock must have extended over 

 English latitudes before the formation of the German 

 Ocean, for they could not have crossed the salt sea, 

 and been imported in that way. Hence, although 

 geologically speaking these familiar objects have 

 been in existence, in England at least, only a com- 

 paratively short time, if we reckoned their occupation 

 in numbers of years we should nevertheless be 

 startled at the amount. Consider the great physical 

 changes that have taken place since they extended 

 hither the depression of the lowest levels on the 

 eastern side of England to form the German Ocean, 

 the submergence of the great plain to the west, to 

 form the Irish Sea. Both these seas are shallow, 

 and indicate recent formation. Arid the occurrence 



