Note 21 — Chapter III · CAUSES OF VARIATION.

Considering that I have already detected more than one thousand species in those islands, it may perhaps be questioned whether the same truth is to be gathered from the result of my Madeiran researches. I would wish it therefore to be understood, first, that my statement refers to that group as contrasted with countries in a similar latitude; and, secondly, that its real fauna is alone taken into account,—the host of introductions from more northern regions, a large proportion of which have probably taken place within a very recent period (as may be fairly presumed from the knowledge that fresh arrivals, an almost necessary consequence of the importation of plants, are occurring nearly every season), having been dismissed from our present inquiry.