VARIATION UNDER NATURE 83 



United States, has described what he calls Phytophagio 

 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 ob- 

 served 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 

 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 an- 

 other, 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 dis- 

 tinct 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 like- 

 wise necessarily occurs with closely allied organisms, 

 which inhabit distinct continents or islands. When, on 

 the other hand, an animal or plant ranges over the same 

 continent, or inhabits many islands in the same archipel- 

 ago, and presents different forms in the different areas, 

 there is always a good chance that intermediate forms 

 will be discovered which will link together the ex- 



