36 THE ORIGIN OF SPECIES 



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 grada- 

 tions, it is probable that they were once thus connected. Mr. Wal- 

 lace, for instance, describes a certain butterfly which presents in 

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

 mediate 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 dif- 

 ferent kinds of females and three or even six different kinds of 

 males. Nevertheless these cases are only exaggerations of the com- 

 mon 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 char- 

 acter of species, but which are so closely similar to other forms, or 

 are so closely linked to them by intermediate gradations, that nat- 

 uralists do not like to rank them as distinct species, are in several 

 respects the most important for us. We have every reason to be- 

 lieve 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 intermediate links; nor will the commonly assumed 

 hybrid nature of the intermediate forms always remove the dif- 

 ficulty. In very many cases, however, one form is ranked as a 



