The relations between the type and the various sub-types, between the type and the individual, between the sub-type and the individual--whether in plant or beast or man--are matters which could not be handled within the limits of this book, and which I have therefore as far as possible ignored.

Nor have I attempted to deal with the difficult problems that are presented by the existence of races, such as the Negro, which seem to be far below the normal level of human development.

There is, however, in the vast region of thought which these and kindred problems open out to us, one by-way which I must be allowed to follow for a while.
The wild _bullace_ is, I believe, the ancestor of many of our yellow _plums_.

In other words, bullacehood can develop into plumhood, and even into the perfection of plumhood.

Similarly human nature can develop into something so high above the normal level of human nature that it might almost seem to belong to another _genus_.