It would never have been expected that the branching of the main nerves close to the great central ganglion of an insect would have been variable in the same species; it might have been thought that changes of this nature could have been effected only by slow degrees; yet Sir J.Lubbock has shown a degree of variability in these main nerves in Coccus, which may almost be compared to the irregular branching of the stem of a tree.

This philosophical naturalist, I may add, has also shown that the muscles in the larvae of certain insects are far from uniform.

Authors sometimes argue in a circle when they state that important organs never vary; for these same authors practically rank those parts as important (as some few naturalists have honestly confessed) which do not vary; and, under this point of view, no instance will ever be found of an important part varying; but under any other point of view many instances assuredly can be given.
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.