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.