Origin of Species Variorum

... 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 great differences of structure, ... as in the two sexes of various animals, in the two or three castes of sterile females or workers amongst insects, and .. in the immature and larval states of many of the lower animals. There are, however, other cases, namely of dimorphism and trimorphism, which might easily be, and have frequently been, confounded with variability, but which are quite distinct. I refer to the two or three different forms, which certain animals of either sex, and certain hermaphrodite plants, habitually present. Thus, Mr. Wallace, who has lately called special 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 together by intermediate varieties. The winged and frequently wingless states of so many Hemipterous insects may probably be included as a case of dimorphism, and not of mere variability. Fritz Müller, also, has recently described analogous but more extraordinary cases with the males of certain Brazilian Crustaceans: thus, the male of a Tanais regularly occurs under two widely different forms, not connected by any intermediate links; one of these forms has much stronger and differently shaped pincers for seizing the female, and the other, as if for compensation, has antennæ much more abundantly furnished with smelling-hairs, so as to have a better chance of finding the female. ↑ Again, the males of another Crustacean, an Orchestia, occur under two distinct forms, with pincers differing much more from each other in structure, than