32 INDIVIDUAL DIFFERENCES. [CHAP. H 



selection to act on and accumulate, in the same manner as man 

 accumulates in any given direction individual differences in his 

 domesticated productions. These individual differences generally 

 affect what naturalists consider unimportant 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 naturalist would be sur- 

 prised 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 systematists 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 bc^n 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 th^ irregular branching 

 of the stem of a tree. This philosophical naturalist, I may add, 

 has also shown that the muscles in the larvae oi 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 vary- 

 ing ; 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. We may instance Rubus, Rosa, and Hieracium 

 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, 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 variability is independent of the conditions of life. 

 I am inclined 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 been seized cm 



