43 - INDIVIDUAL DIFFERENCES. 



shells, at former periods of time. These facts are very 

 perplexing, for they seem to show that this kind of varia- 

 bility is independent of the conditions of life. I am in- 

 clined 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.^ in- 

 dependently 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 tritnorphism, 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, reg- 

 ularly appear under two or even three conspicuously dis- 

 tinct forms, not connected by intermediate varieties. Fritz 

 Muller 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 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, describes a certain butterflj^ which presents in 

 the same island a great range of varieties connected by in- 

 termediate links, and the extreme links of the chain closely 

 resemble the two forms of an allied dimorphic species in- 

 habiting 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 varieties. 

 So it is, as I have myself observed, with some dimorphic 

 plants. It certainly at first appears a highly remarkable 

 fact that the same female butterfly should have the power 

 of producing at the same time three distinct female forms 

 and a male; and that an hermaphrodite plant should pro- 



