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 distin- 

 guished from each other by a greater amount of difEer- 

 ence 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 charatiterised as varieties in Mr. Wollaston's ad- 

 mirable 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 zo- 

 ologists. 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 America and Europe 



