This might have been anticipated, for the mere fact of many species of the same genus inhabiting any country, shows that there is something in the organic or inorganic conditions of that country favourable to the genus; and, consequently, we might have expected to have found in the larger genera, or those including many species, a larger proportional number of dominant species.

But so many causes tend to obscure this result, that I am surprised that my tables show even a small majority on the side of the larger genera.

I will here allude to only two causes of obscurity.