60 DOUBTFUL SPECIES. [Chap. II 



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 Phy- 

 tophagic 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 in- 

 termediate links connecting the several forms should 

 now be found. The naturalist thus loses his best guide 

 in determining whether to rank doubtful forms as varie- 

 ties or species. This likewise necessarily occurs with 

 closely allied organisms, which inhabit distinct conti- 

 nents or islands. When, on the other hand, an animal 



