54 INDIVIDUAL DIFFERENCES. Chap. II. 



perienced naturalist would be surprised at the number of the 

 oases of variability, even in important parts of structure, which 

 lie could collect on ^ood authority, as I have collected, during 

 a course of years. It should be remembered that systematists 

 are far from bcing^ pleased at finding variability in important 

 characters, and that there are not many men who wiU labori- 

 ously examine internal and important ort^ans, and compare 

 tliem in many specimens of the same species. It Avould 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, recently, Sir J. Lubbock has shown a degree of . 

 variability in these main nerves in Coccus, which ma}' almost 

 be compared to the irregular branching of the stem of a tree. 

 This philosophical naturalist, I may add, has also recently 

 sliowu that the muscles in the larva? of certain insects are far 

 from luiiform. 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 natu- 

 ralists have honestly confessed), which do not vary ; and, under 

 this point of view, no instance will ever be found of an impor- 

 tant 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 the 

 species present an inordinate amount of variation ; and about 

 which hardly two naturalists agree whether to rank them as 

 species or as varieties. We niay instance Rubus, Rosa, and 

 Hieracium among plants, several genera of insects, several gen- 

 era of Brachiopod shells, and the Ruff (Machetes pugnax) 

 among birds. In most polpnorphic genera some of the species 

 have fixed and definite characters. Genera which are poly- 

 morpliic in one country seem to be, with some few exceptions, 

 ])olymorphic in other countries, and likewise, judging from 

 Brachiopod shells, at former periods of time. These facts are 

 very perplexing, for they seem to sliow that this kind of varia- 

 bility is independent of the conditions of life. I am inclined 

 to suspect that we have, at least in some of these polymorphic 

 gen(»ra, variations which are of no service or disservice to the 

 Hpccics, and which, consequently, have not been seized on and 

 reiiden^d delinite by natural selection, as hereafter to be ex- 

 j)hiined. 



