CHAP. II.] DOUBTFUL SPECIES. 35 



Lepidoptera, inhabiting the islands of the great Malayan archi- 

 pelago, shows that they may be classed under four heads, namely, 

 as variable forms, as local forms, as geographical races or sub- 

 species, and as true representative species. The first or variable 

 forms vary much within the limits of the same island. The local 

 forms are moderately constant and distinct in each separate 

 island ; but when all from the several islands are compared 

 together, the differences are seen to be so slight and graduated, 

 that it is impossible to define or describe them, though at the same 

 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. 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 distinction between species and varieties. On the islets of 

 the little Madeira group there are many insects which are cha- 

 racterised 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 

 Phytophagic species. Most vegetable-feeding insects live on one 

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



