Chap. II.] DOUBTFUL SPECIES. 59 



strongly marked and important characters, " there is no 

 possible test but individual opinion to determine which 

 of them shall be considered as species and which as 

 varieties." Lastly, representative species fill the same 

 place in the natural economy of each island as do the 

 local forms and sub-species; but as they are dis- 

 tinguished 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 recognised. 



Many years ago, when comparing, and seeing others 

 compare, the birds from the closely neighbouring 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 dis- 

 tinction between species and varieties. On the islets 

 of the little Madeira group there are many insects 

 which are characterised 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 

 same zoologists. Several experienced 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 between 

 the homes of two doubtful forms leads many naturalists 

 to rank them as distinct species ; but what distance, it 

 has been well asked, will suffice; if that between 



