46 DOUBTFUL SPECIES. 



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 varie- 

 ties; 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 can not 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 inter- 

 mediate 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 pre- 

 sent varieties; but then these same naturalists rank the 

 slightest difference as of specific value; and when the same 

 ideiitical form is met with in two distant countries, or in 

 two geological formations, they believe that two distinct 

 species are hidden under the same dress. The term 

 species thus comes to be a mere useless abstraction, imply- 

 ing 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 compe- 

 tent 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 doubt- 

 ful species well deserve consideration; for several interest- 

 ing lines of argument, from geographical distribution, 

 analogical variation, hybridism, etc., have been brought to 

 bear in the attempt to determine their rank; but space 



