INDIVIDUAL DIFFERENCES 61 



tries, 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 con- 

 sequently 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 female 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 Ma- 

 layan 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 males of certain Brazilian 

 Crustaceans: thus, the male of a Tanais regularly occurs 

 under two distinct forms; one of these has strong and dif- 

 ferently 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, describes a certain butterfly which 

 presents in the same island a great range of varieties con- 

 nected by intermediate links, and the extreme links of the 

 chain closely resemble the two forms of an allied dimorphic 

 species inhabiting another part of the Malay archipelago. 

 Thus also with ants, the several worker-castes are generally 

 quite distinct; but in some cases, as we shall hereafter see, 

 the castes are connected together by finely graduated varie- 

 ties. So it is, as I have myself observed, with some dimor- 

 phic plants. It certainly at first appears a highly remarkable 

 fact that the same female butterfly should have the power 



