I see no reason to limit the process of modification, as now explained, to the formation of genera alone.

If, in the diagram, we suppose the amount of change represented by each successive group of diverging dotted lines to be great, the forms marked a14 to p14, those marked b14 and f14, and those marked o14 to m14, will form three very distinct genera.

We shall also have two very distinct genera descended from (I), differing widely from the descendants of (A).