THE RIG IX OF GROL'SE. 199 



be transported across wide stretches of intcrvcninj;^ 

 water. 



Mr, Wallace has noticed that in all such insulated 

 lands there is a great tendency for species to var}-, 

 partly throuLjh the si)ecial sets of circumstances to which 

 they are thus exposed, and partly through the rarity of 

 crosses with the original stock, which doubtless continues 

 to develop and alter on its own part in another direction, 

 under pressure of other influences to which it is exposed 

 in the wider continents where it dwells. In Ihitain, 

 though so recently separated from the mainland, as Mr. 

 Wallace points out, this tendency has already produced 

 a few very marked effects. An immense number of our 

 native plants appear in slightly different varieties from 

 those of the mainland ; and in our outlying islands, 

 such as Man, Wight, Lundy, Arran, and the Hebrides, 

 such variation is exceptionally common. Among insects 

 we have several British species ; among fish we have six 

 or seven kinds of trout ; and among birds we have the 

 grouse, which is quite unknown in any other part of the 

 world. Its nearest Continental representative is the 

 willow-grouse of Scandinavia, which ranges all round 

 the northern hemisphere even up to the Pole. But the 

 willow-grouse changes its coat to white in v/intcr, like 

 the ptarmigan, whereas the Scotch red grouse keeps its 

 summer dress the whole year round : and many minor 

 points of difference have caused our own bird to be 

 universally ranked by naturalists as a good species. 

 Ought we really to regard it as the primitive type from 

 which the Continental bird is derived, or ought we 

 rather to consider it as a special insular descendant (jf 

 the willow-grousCj or ought we finally to look upon both 



