82 THE ORIGIN OF SPECIES 



species fill the same place in the natural economy of each 

 island as do the local forms and sub-species; bat as they 

 are distinguished from each other by a greater amount 

 of difference than that between the local forms and sub- 

 species, they are almost universally ranked by naturalists 

 as true species. Nevertheless, no certain criterion can 

 possibly be given by which variable forms, local forms, 

 sub-species, and representative species can be recognized. 



Many years ago, when comparing and seeing others 

 compare, the birds from the closely neighboring islands 

 of the Galapagos archipelago, one with another, and with 

 those from the American mainland, I was much struck 

 how entirely vague and arbitrary is the distinction be- 

 tween species and varieties. On the islets of the little 

 Madeira group there are many insects which are charac- 

 terized as varieties in Mr. Wollaston's admirable work, 

 but which would certainly be ranked as distinct species 

 by many entomologists. Even Ireland has a few animals, 

 now generally regarded as varieties, but which have been 

 ranked as species by some zoologists. Several experi- 

 enced ornithologists consider our British red grouse as 

 only a strongly -marked race of a Norwegian species, 

 whereas the greater number rank it as an undoubted 

 species peculiar to Great Britain. A wide distance be- 

 tween the homes of two doubtful forms leads many nat- 

 uralists to rank them as distinct species; but what dis- 

 tance, it has been well asked, will suffice; if that between 

 America and Europe is ample, will that between Europe 

 and the Azores, or Madeira, or the Canaries, or between 

 the several islets of these small archipelagos, be sufi&- 

 cient? 



Mr. B. D. Walsh, a distinguished entomologist of the 



