judging from Brachiopod shells, at former periods of time. These facts are
very perplexing, for they seem to show that this kind of variability is
independent of the conditions of life. I am inclined to suspect 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, independently 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
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 Muller 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 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 possible 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 Archipelago. Thus also with ants, the several worker-castes are
generally quite distinct; but in some cases, as we shall hereafter see, the