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.