46 VARIATION UNDER NATURE. Chap. II. 



by slow degrees : yet quite recently Mr. 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 quite recently shown 

 that the muscles in the larvae 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 cha- 

 racter 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 natu- 

 ralists 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 cha- 

 racters. 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 perplexing, for they seem to show that this kind 

 of variability is independent of the conditions of life. 

 I am inclined to suspect that We see in these poly- 

 morphic 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. 



