xxxiv INTEODUCTION 



taken in conjunction -with the occurrence in the lake of the shrimp 

 Mysis relicta, a northern animal belonging to a genus essentially 

 marine, and with the fact that the Common Mussel [Mytilus edulis) 

 is found in a fossil state in Boulder-clay on the shores, one is 

 tempted to look on these plants as survivals of a time when a 

 lower level of the land, towards the close of the Glacial Epoch, 

 caused marine conditions to prevail. 



Human Influence on the Flora. 



It is essential that we should bear in mind the profound 

 influence which the operations of man have exercised on the 

 flora of the country. Hardly a spot but bears evidence of the 

 long human occupation of our island. It is not easy to con- 

 jecture the primeval condition of the fertile portions of the country, 

 before tillage, grazing, and drainage began to play their part. 

 We can conceive great woods and thickets, open park-like land, and 

 grassy downs, but the details of the primitive vegetation we may 

 never know. Where tillage has not reached, clearing and grazing 

 have altered the face of the country, so that marshes and mountain- 

 tops, glens and sand-dunes, all bear the stamp of man's interference. 

 Only perhaps in the centre of the great bogs, on inaccessible cliffs, 

 or in the larger lakes, do we find a flora absolutely undisturbed. In 

 the wilder and rougher parts of the country, it is probable that 

 the bulk of the original flora is still to be found ; but it cannot be 

 doubted that the plant-associations of the fertile grounds have been 

 scattered to the winds. There is a difficulty in tracing the extinc- 

 tion of plants, which must have gone on slowly through centmies. 

 The most conspicuous example in Ireland is the Scotch Fir, which 

 at one time, as our bogs tell us, covered vast tracts of country, and 

 was probably finally exterminated by the demand for fuel for 

 smelting and domestic pm-poses. The destruction of arboreal vege- 

 tation by man or by grazing animals is likewise made clear in 

 wild districts like Connemara, where not a native tree is to be seen 

 for miles, save on islands in lakes, where the belt of water has 

 preserved fragments of the old flora, and we find an abundant 

 growth of Holly, Birch, Mountain- Ash, Yew, Oak, and Juniper. A 

 recent example of plant-destruction by drainage may be studied at 



