51 INDIVIDUAL DIFFEKENCES. [Cfap. H. 



men who will laboriously examine internal and 

 important organs, and compare them in many specimens 

 of the same species. It would never have been ex- 

 pected 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 

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

 in the larvae 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 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 the 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. * AVe may instance 

 Paibus, Eosa, and Hieracmm amongst 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, 



