60 DOUBTFUL SPECIES. [Chap. II. 



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 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 colour, 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 strongly 

 marked, and when both sexes and all ages are affected, 

 the forms are ranked by all entomologists as good 

 species. But no observer can determine for another, 

 even if he can do so for himself, which of these 

 Phytophagic forms ought to be called species and 

 which varieties. Mr. Walsh ranks the forms which it 

 may be supposed would freely intercross, as varieties ; 

 and those which appear to have lost this power, as 

 species. As the differences depend on the insects 

 having long fed on distinct plants, it cannot be 

 expected that intermediate links connecting the several 

 forms should now be found. The naturalist thus loses 

 his best guide in determining whether to rank doubtful 

 forms as varieties or species. This likewise necessarily 

 occurs with closely allied organisms, which inhabit 

 distinct continents or islands. AVhen, on the other 



