Page - 65 - in The Origin of Species

DOUBTFUL SPECIES 65 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 differ- ences 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 deter- mine 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 connect- ing 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 neces- sarily 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 in- habits 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 of specific value ; and when the same identical form is met with in two distinct countries, or in two geologi- cal formations, they believe that two distinct species are hid- den under the same dress. The term species thus comes to be a mere useless abstraction, implying and assuming a sep- arate act of creation. It is certain that many forms, consid- ered 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. c—lie XI