There is one point connected with individual differences which is extremely perplexing: I refer to those genera which have been called "protean" or "polymorphic," in which species present an inordinate amount of variation.

With respect to many of these forms, hardly two naturalists agree whether to rank them as species or as varieties.

We may instance Rubus, Rosa, and Hieracium among plants, several genera of insects, and of Brachiopod shells.

In most polymorphic genera some of the species have fixed and definite characters.