VARIATION UNDER NATURE 43 



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 quite 

 recently shown that the muscles in the larvaB of certain 

 insects are very far from uniform. Authors sometimes 

 argue in a circle when they state that important organs 

 never vary ; for these same authors practically rank 

 that character as important (as some few naturalists 

 have honestly confessed) which does not vary ; and, 

 under this point of view, no instance of an important 

 part varying will ever be found : but under any other 

 point of view many instances assuredly can be given. 



There is one point connected with individual differ- 

 ences which seems to me extremely perplexing : I refer 

 to those genera which have sometimes been called 

 ' protean ' or ' polymorphic,' in which the species 

 present an inordinate amount of variation ; and hardly 

 two naturalists can agree which forms to rank as 

 species and which as varieties. We may instance 

 Rubus, Rosa, and Hieracium amongst plants, several 

 genera of insects, and several genera 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 

 some few exceptions, polymorphic in other countries, 

 and likewise, judging from Brachiopod shells, at former 

 periods of time. These facts seem to be very per- 

 plexing, for they seem to show that this kind of varia- 

 bility is independent of the conditions of life. I am 

 inclined to suspect that we see in these polymorphic 

 genera variations in points of structure which are of no 

 service or disservice to the species, and which con- 

 sequently have not been seized on and rendered 

 definite by natural selection, as hereafter will be 

 explained. 



Those forms which possess in some considerable 

 degree the character of species, but which are so 

 closely similar to some other forms, or are so closely 

 linked to them by intermediate gradations, that 

 naturalists do not like to rank them as distinct species, 



