INDIVIDUAL DIFFERENCES. 41 



consider imimportant parts; but I could show, by a long 

 catalogue of facts, that parts which must be called important, 

 whether viewed under a physiological or classificatory point 

 of view, sometimes vary in the individuals of the same 

 species. I am convinced that the most experienced natu- 

 ralist would be surprised at the number of the cases of 

 variability, even in important parts of structure, which he 

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

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

 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 philo- 

 sophical naturalist, I may add, has also shown that the 

 muscles in tiie larvae of certain insects are far from uni- 

 form. 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 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 Briichiopod 



