40 INDIVIDUAL Dlftfl&fcfitfCIS. 



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 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 varieties. Fritz Muller has described anal- 

 ogous but more extraordinary cases with the males of cer- 

 tain Brazilian Crustaceans : thus, the male of a Tanais regu- 

 larly occurs under two distinct forms ; one of these has strong 

 and differently shaped pincers, and the other has anten- 

 nae 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 cer- 

 tain butterfly which presents in the same island a great 

 range of varieties connected 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 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 produce from the same 

 seed-capsule three distinct hermaphrodite forms, bearing 

 three different kinds of females and three or even six dif- 

 ferent kinds of males. Nevertheless these cases are only 

 exaggerations of the common fact that the female produces 

 offspring of two sexes which sometimes differ from each 

 other in a wonderful manner. 



