Chap. U.] INDIVIDUAL DIFFERENCES 35 



the same species. It would never Lave 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 naturalist, I may add, has also shown that the 

 muscles in the larva? 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 inor- 

 dinate 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 Eubus, Kosa, 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 like- 

 wise, 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 on and rendered definite 

 by natural selection, as hereafter to be explained. 



Individuals of the same species often present, as is known to 

 every one, great differences of structure, independently of variation, 

 as in the two sexes of various animals, in the two or three castes of 

 sterile females or workers amongst insects, and in the immature and 

 larval states of many of the lower animals. There are, also, cases 

 of dimorphism and trimorphism, both with animals and plants. 

 Thus, Mr. Wallace, who has lately called attention to the subject, 

 has shown that the females of certain species of butterflies, in the 

 Malayan archipelago, regularly appear under two or even three 

 conspicuously distinct forms, not connected by intermediate varieties. 

 "Fritz Miiller has described analogous but more extraordinary cases 



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