36 DOUBTFUL SPECIES. [Chap. II. 



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 

 antennaa 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 pre- 

 sents 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 pro- 

 ducing 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. 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. 



Doubtful Species. 



The forms which possess in some considerable degree the cha- 

 racter of species, but which are so closely similar to other forms, 01 

 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 

 forms have permanently retained their characters for a long 

 time; for as long, as far as we know, as have good and true 

 species. Practically, when a naturalist can unite by means of 

 intermediate links any two forms, he treats the one as a variety 

 of the other ; ranking the most common, but sometimes the one 

 first described, as the species, and the other as the variety. 

 But cases of great difficulty, which I will not here enumerate, 

 sometimes arise in deciding whether or not to rank one form as a 

 variety of another, even when they are closely connected by inter- 

 mediate links ; nor will the commonly-assumed hybrid nature oi 



