38 THE ORIGIN OF SPECIES 



ficiently 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 differ- 

 ence 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 

 recognized. 



Many years ago, when comparing, and seeing others compare, 

 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 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 character- 

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

 would certainly be ranked as distinct species by many entomolo- 

 gists. Even Ireland has a few animals, now generally regarded as 

 varieties, but which have been ranked as species by some zoolo- 

 gists. 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 pe- 

 culiar 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 archipelagoes, 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 indiscriminately on 

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

 ever, 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, have been observed thus 

 to differ in a slight degree. When the differences are rather more 



