CiiAP. II. INDIVIDUAL DIFFERENCES. 55 



Individuals of the same species often present ^cat difTcr- 

 eiices of structure, as in the two sexes of various animals, iuthe 

 two or three castes of sterile females or workers amon^^ insects, 

 and in the immature and larval states of many of the lower ani- 

 mals. Tlierc are, however, other cases, namely, of dimori)hism 

 and trimorphism, which might easily be, and have frecjucntly 

 l)een, confounded with varial^ility, but which are quite distinct. 

 I refer to the two or three dillerent forms which certain animals 

 of either sex, and certain hermaphrodite plants, habitually pre- 

 sent. Thus, Mr. Wallace, who has lately called special atten- 

 tion to the subject, has shown that the females of certain spe- 

 cies of butterflies, in the Malayan archipelaj^o, regularly ap- 

 pear under two or even three consjoicuously distinct forms, not 

 connected together by intermediate varieties. The winged and 

 frequently wingless states of so many Hemipterous insects may 

 probably be included as a case of dimorphism, and not of mere 

 variability. Fritz Muller, also, has recently described analo- 

 gous but more extraordinary cases with the males of certain 

 Brazilian Crustaceans : thus, the male of a Tanais regularly 

 occurs under two widely-dillerent forms, not connected by any 

 intermediate links ; one of these forms has much stronger and 

 differently-shaped pincers for seizing the female, and the other, 

 as if for compensation, has anlennaj much more abundantly 

 furnished with smelhng-hairs, so as to have a better chance of 

 finding the female. Again, the males of another Crustacean, 

 an Orchestia, occur under two distinct forms, with pincers dif- 

 fering much more from each other in structure, than do the 

 pincers of most species of the same genus. With respect to 

 plants, I have recently shown that in several -widely-distinct 

 orders, the species present two or even three forms, which arc 

 abruptly distinguished from each other in several important 

 ]ioints, as in the size and color of the pollen-grains ; and these 

 forms, though all hermaphrotlites, differ from each other in 

 their reproductive power, so that for full fertility, or indeed in 

 some cases for any fertility, they must reciprocally impregnate 

 each other. Although the forms of the few dimorphic and tri- 

 morphic animals and plants which have been studied, are not 

 now connected together by intermi^diate links, it is ]irol)able 

 that thi^ will be found to occur in other cases ; for Mr. \\''allacc 

 observed a certain Ijutterlly which jiresentcd in the same island 

 a great range of varieties connected by intermediate links, and 

 the extreme links of the chain closely resembled the two forms 

 of an allied dimorphic species inhabiting another part of the 



