DOUBTFUL BPEUIKS. 45 



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. Nevertheless, no certain 

 criterion can possibly be given by which variable forms, 

 local forms, sub species and representative species can bo 

 recognized. 



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

 pare, 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 en- 

 tirely vague and arbitrary is the distinction between species 

 and varieties. On the islets of the little Maderia group 

 there are many insects which are characterized as 

 varieties in Mr. Wollaston's admirable work, but which 

 would certainly be ranked as distinct species by many en- 

 tomologists. Even Ireland has a few animals, now gen- 

 erally regarded as varieties, but which have been ranked as 

 species by some zoologists. Several experienced ornitholo- 

 gists 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 pecuhar to Great 

 Britain. A wide distance between the homes of two doubt- 

 ful forms leads many naturalists to rank them as distinct 

 species; but what distance, it has been well asked, will suf- 

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

 between Europe and the Azores, or Maderia, or the Cana- 

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

 be sufficient? 



Mr. B. D. "Walsh, a distinguished entomologist of tlie 

 United States, has 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 indiscriminately 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 color, 

 size, or in the nature of their secretions. In some instances 

 the males alone, in other instances, both males and females. 



