38 DOUBTFUL SPECIES. [Chap. It 



ferences are seen to be so slight and graduated, that it is impossible 

 to define or describe them, though at the samu time the extreme 

 forms are sufficiently distinct. The geographical races or sub-species 

 are local forms completely fixed and isolated ; but as they do not 

 differ from each other by 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 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. Never- 

 theless, 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 archi- 

 pelago, one with another, and with those from the American main- 

 land, I was much struck how entirely vague and arbitrary is the 

 distinction between 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 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 is ample, 

 will that between Europe and the Azores, or Madeira, or the 

 Canaries, or between the several islets of these small archipelagos, 

 be sufficient ? 



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

 States, has described what he calls Phytophagic varieties and Phy- 

 tophagic species. Most vegetable-feeding insects live on one kind 

 of plant or on one group of plants ; some feed mdiscrirninately on 

 many kinds, but do not in consequence vary. In several cases, 

 however, insects found living on different plants, have been observed 

 by Mr. Walsh to present in their larval or mature state, or in both 

 states, slight, though constant differences in colour, size, or in the 



