CHAP. II.J DOUBTFUL SPECIES. 33 



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 varia- 

 tion, 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 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 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 smell ing-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 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 archi- 

 pelago. 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 different kinds of males. Never- 

 theless 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. 



Doubtful Species. 



The forms which possess in some considerable degree the 

 character of species, but which are so closely similar to other 

 forms, or are so closely linked to them by intermediate gradations, 

 that naturalists do not like to rank them as distinct species, are 

 in several respects the most important for us. We have every 

 reason to believe that many of these doubtful and closely allied 



