All the descendants of the genus F, along its whole line of descent, are supposed to have been but little modified, and they form a single genus.

But this genus, though much isolated, will still occupy its proper intermediate position.

The representation of the groups as here given in the diagram on a flat surface, is much too simple.

The branches ought to have diverged in all directions.

If the names of the groups had been simply written down in a linear series the representation would have been still less natural; and it is notoriously not possible to represent in a series, on a flat surface, the affinities which we discover in nature among the beings of the same group.