n6 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 " Type" " 



