398 INFLUENCES OF CULTIVATION 



Brown Rats and Rabbits, which abound on very many 

 Scottish islands, must have been borne thither in most cases 

 by human modes of transport, as have been those intro- 

 ductions from foreign lands which shall be considered in 

 another chapter. 



Roads and bridges are important factors in the dispersal 

 of animal life. Many creatures prefer the easy means of 

 communication man has prepared for his own comfort, and 

 have no hesitation in turning to their own use objects planned 

 by him for quite another purpose. In moving from one wood 

 to another, or from one district to another, Squirrels prefer to 

 run along roads, the open rides in forests, the top of a fence 

 or stone dyke. They have been known to make regular use 

 of dykes in passing between coverts. The Common Brown 

 Hare is said (New Statistical Account) to have appeared in 

 the mountainous districts of Lismore and Appin "not until 

 after roads were made which opened communication with 

 the low country." 



In many cases where rivers blocked the way of progress 

 for certain animals, man has bridged the barrier and given 

 them easy access to new districts. Rats and Squirrels, ex- 

 cellent swimmers though they are, make frequent use of 

 bridges, and Dr Harvie- Brown, from his examination of 

 dates and places of appearance, was of opinion that the first 

 invasion of Perthshire by Squirrels from Stirlingshire was 

 by the Bridge of Frew over the Forth ; and that they probably 

 made their first appearance in Sutherland in 1859 over Bonar 

 Bridge, although it was not until the railway bridge at In- 

 vershin was built in 1869 that Squirrels became plentiful in 

 the East of Sutherland. I have already suggested that 

 Dr Harvie- Brown laid too much stress on the necessity of 

 the Invershin Bridge for migration, but there can be little 

 doubt that here and elsewhere bridges have to a considerable 

 extent aided in the dispersal of animals. 



The great transport systems which cross and cross again 

 the countries of the world have also played their part in 

 aiding the dispersal of the beasts of the field and birds of the 

 air as well as of the sons of men. Apart from the influence 

 of the rolling stock of our railways in transporting creatures 

 accidentally or otherwise, the permanent iron way and its 

 embankments have acted as a high road along which many 



