Chap. II.] INDIVIDUAL DIFFERENCES. 55 



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 

 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 with the maj.es of certain Brazilian Crustaceans : 

 thus, the male of a Tanais regularly occurs under two 

 distinct forms ; one of these has strong and differently 

 shaped pincers, and the other has antennae much more 

 abundantly furnished with smelling-hairs. Although 

 in most of these cases, the two or three forms, both 

 with animals and plants, are not now connected by 

 intermediate gradations, it is probable that they were 

 once thus connected Mr. Wallace, for instance, 



