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.