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.