ii6 SECRETS OF EARTH AND SEA 



we are justified in considering a few isolated intermediate 

 forms (which often occur connecting two greatly-differing 

 species) as survivors of a former complete graduated 

 series of intermediate forms, which came into existence 

 by slow modification of an ancestral stock, and may, when 

 the stock was widely spread over a continental area, not 

 merely have succeeded one another in time, but actually 

 coexisted in neighbouring regions. 



There are many remarkable facts bearing upon the 

 origin of "species," the description of which fills volumes 

 written by such men as Darwin, Wallace, Poulton, and 

 others, and become interesting to every one who has 

 gained a correct notion of what naturalists mean by a 

 " species." I will cite one in order to illustrate this. The 

 bird which we call the red grouse, or nowadays simply 

 " grouse " (the old Scotch name for it was " muir-fowl "), is 

 one of twenty-four birds (among the 400 species of birds 

 which live in the British Islands), including several kinds of 

 titmouse, the goldfinch, bullfinch, song-thrush, stonechat. 

 jay, dipper, and others which are very closely similar to 

 species of birds living in Continental Europe, yet show 

 some definite and constant marks, such as small differences 

 in the colour of a group of feathers, enabling us to dis- 

 tinguish the British from the Continental forms. Are these 

 twenty-four British forms to be regarded as distinct species ? 



The red grouse is placed in a genus called " Lagopus," 

 of which there are several species in the northern hemi- 

 sphere. In Scotland the red grouse, which is distinguished 

 as Lagopus Scoticus, is accompanied by a rarer species 

 of Lagopus, which lives in high, bare regions. This is 

 the bird called by the Celtic name " ptarmigan " ; it 

 differs in several points from the red grouse, and acquires 

 white plumage in the winter, which the latter bird does 

 not; it is called Lagopus mutus. Now in Norway we 

 find also two species of grouse or Lagopus, called " 



