VARIATION UNDER NATURE 35 



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 de- 

 grees; yet Sir J. Lubbock has shown a degree of variability in these 

 main nerves in Coccus, which may almost be compared to the irreg- 

 ular 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 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 variability is independent of the conditions of life. I 

 am inclined to suspect that we see, at least in some of these poly- 

 morphic genera, variations which are of no service or disservice to 

 the species, and which consequently have not been seized on an4 

 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 among 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 appeared under two or even three 

 conspicuously distinct forms, not connected by intermediate vari- 

 eties. Fritz Muller has described analogous but more extraordinary 



