INDIVIDUAL DIFFERENCES. 39 



species. I am convinced that the most experienced natur- 

 alist would be surprised at the number of the cases he 

 variability, even in important parts of structure, which of 

 could collect on good authority, as I have collected, during 

 a course of years. It should be remembered that system a- 

 tists are far from being pleased at finding variability in 

 important characters, and that there are not many men 

 who will laboriously examine internal and important 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 natur- 

 alist, I may add, has also shown that the muscles in the 

 larvas 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 hon- 

 estly 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. We 

 may instance Rubus, Rosa, 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 periods of time. These facts are very 

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

 bility is independent of the conditions of life. I am in- 

 clined to suspect that we see, at least in some of these 

 polymorphic genera, variations which are of no service or 

 disservice to the species, and which consequently have not 



