58 DOUBTFUL SPECIES. Cuap. II. 



individual opinion to determine which of them shall be consid- 

 ered as species and -which as varieties." Lastly, representa- 

 tive species fill the same place in the natural economy of each 

 island as do the local forms and sub-species ; but, as they are 

 distinp^uished from each other by a greater amount of differ- 

 ence than that between the local forms and sub-species, they 

 are almost imiversally ranked by naturalists as true species. 

 Nevertheless, no certain criterion can possibly be given by 

 which variable forms, local forms, sub-species, and representa- 

 tive species can be recognized. 



Many years ago, when comparing, and seeing others com- 

 pare, the birds from the closely-neighboring islands of the 

 Galapagos archipelago, both 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 Avhich are characterized as varieties in Mr. Wol- 

 laston'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 Avell asked, Avill suf- 

 fice ; if that between America and Europe is ample, will that 

 lietween Europe and the Azores, or Madeira, or tlie Canaries, 

 or between the several islets of these small archipelagos, be 

 sufficient ? 



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

 States, has lately described what he calls Phytophagic varieties 

 and Phytophagic species. Most vegetable-feeding insects live 

 on one kind of plant or on one group of plants ; some feed in- 

 discriminately 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, either ex- 

 el usivel}' in their larval or mature state, or in both states, 

 slight though constant differences in color, size, or in the nature 

 of their secretions. In some instances the males alone, in other 

 instances l)()th males and females, liave been olisen'ed to be 

 thus alTected in a shght degree. When the diflerencos are 



