78 THE ORIGIN OF SPECIES 



periods of time. These facts are very perplexing, for 

 they seem to show that this kind of variability is inde- 

 pendent of the conditions of life. I am inclined to sus- 

 pect that we see, at least in some of these polymorphic 

 genera, variations which are of no service or disservice 

 to the species, and which consequently have not 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, inde- 

 pendently 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 at- 

 tention to the subject, has shown that the females of 

 certain species of butterflies, in the Malay archipelago, 

 regularly appear under two or even three conspicuously 

 distinct forms, not connected by intermediate varieties. 

 Fritz Miiller has described analogous but more extraordi- 

 nary cases with the males of certain Brazilian Crusta- 

 ceans: thus, the male of a Tanais regularly occurs under 

 two distinct forms; one of these has strong and differ- 

 ently 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 interme- 

 diate 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 ex- 



