36" DOUBTFUL SPECIES. [CHAP. II. 



nately 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. When, on the other hand, an animal or plant ranges 

 over the same continent, or inhabits many islands in the same 

 archipelago, 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 extreme states ; and these 

 are then degraded to the rank of varieties. 



Some few naturalists maintain that animals never present 

 varieties ; but then these same naturalists rank the slightest 

 difference as CA specific value ; and when the same identical form 

 is met with in two distant countries, or in two geological forma- 

 tions, they believe that two distinct species are hidden under the 

 same dress. The term species thus comes to be a mere useless 

 abstraction, implying and assuming a separate act of creation. It 

 is certain that many forms, considered by highly-competent judges 

 to be varieties, resemble species so completely in character, that 

 they have been thus ranked by other highly-competent judges. 

 But to discuss whether they ought to be called species or varieties, 

 before any definition of these terms has been generally accepted, 

 is vainly to beat the air. 



Many of the cases of strongly-marked varieties or doubtful 

 species well deserve consideration ; for several interesting lines 

 of argument, from geographical distribution, analogical variation, 

 hybridism, &c., have been brought to bear in the attempt to 

 determine their rank ; but space does not here permit me to 

 discuss them. Close investigation, in many cases, will no doubt 

 bring naturalists to agree how to rank doubtful forms. Yet it 



