Note 38 — Chapter III · CAUSES OF VARIATION.

I do not think it necessary to apologize for the apparent disposal of this quæstio vexata; because, from the wildness of the upland ridges to which the D. obscuroguttatus is in Madeira exclusively confined, I deem it an absolute impossibility that it could ever have been introduced, through any chance agencies whatsoever. And hence, unless we reject the doctrine of specific centres in toto, I contend that it must have migrated, together with other insects similarly circumstanced, by ordinary means, and without natural impediments, from its own area of diffusion.