VARIATION UNDER NATURE 77 



portant organs, and compare them in many specimens of 

 the same species. 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 con- 

 fessed) which do not vary; and, under this point of view, 

 no instance will ever be found of an important part vary- 

 ing; but under any other point of view many instances 

 assuredly can be given. 



There is one point connected with individual differ- 

 ences which is extremely perplexing: I refer to those 

 genera which have been called "protean" or "polymor- 

 phic," in which the 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 Eubus, Eosa, 

 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. 

 Genera which are polymorphic in one country seem to be, 

 ■with a few exceptions, polymorphic in other countries, 

 and likewise, judging from Brachiopod shells, at former 



