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.

Fresh water and salt-loving plants generally have very wide ranges and are much diffused, but this seems to be connected with the nature of the stations inhabited by them, and has little or no relation to the size of the genera to which the species belong.

Again, plants low in the scale of organisation are generally much more widely diffused than plants higher in the scale; and here again there is no close relation to the size of the genera.

The cause of lowly-organised plants ranging widely will be discussed in our chapter on Geographical Distribution.
From looking at species as only strongly marked and well-defined varieties, I was led to anticipate that the species of the larger genera in each country would oftener present varieties, than the species of the smaller genera; for wherever many closely related species (i.e., species of the same genus) have been formed, many varieties or incipient species ought, as a general rule, to be now forming.