would probably soon be lost, as the males would prevail, which retained
their immature dress for the longest period, and thus the character of
the species would ultimately be modified. (39. Other animals, belonging
to quite distinct classes, are either habitually or occasionally
capable of breeding before they have fully acquired their adult
characters. This is the case with the young males of the salmon.
Several amphibians have been known to breed whilst retaining their
larval structure. Fritz Muller has shewn ( Facts and arguments for
Darwin, Eng. trans. 1869, p. 79) that the males of several amphipod
crustaceans become sexually mature whilst young; and I infer that this
is a case of premature breeding, because they have not as yet acquired
their fully-developed claspers. All such facts are highly interesting,
as bearing on one means by which species may undergo great
modifications of character.) If, on the other hand, the young never
succeeded in obtaining a female, the habit of early reproduction would
perhaps be sooner or later eliminated, from being superfluous and
entailing waste of power.
The plumage of certain birds goes on increasing in beauty during many
years after they are fully mature; this is the case with the train of
the peacock, with some of the birds of paradise, and with the crest and
plumes of certain herons, for instance, the Ardea ludovicana. (40.
Jerdon, Birds of India, vol. iii. p. 507, on the peacock. Dr.
Marshall thinks that the older and more brilliant males of birds of
paradise, have an advantage over the younger males; see Archives
Neerlandaises, tom. vi. 1871. On Ardea, Audubon, ibid. vol. iii. p.
139.) But it is doubtful whether the continued development of such
feathers is the result of the selection of successive beneficial
variations (though this is the most probable view with birds of
paradise) or merely of continuous growth. Most fishes continue
increasing in size, as long as they are in good health and have plenty
of food; and a somewhat similar law may prevail with the plumes of
birds.
CLASS V. WHEN THE ADULTS OF BOTH SEXES HAVE A DISTINCT WINTER AND
SUMMER PLUMAGE, WHETHER OR NOT THE MALE DIFFERS FROM THE FEMALE, THE
YOUNG RESEMBLE THE ADULTS OF BOTH SEXES IN THEIR WINTER DRESS, OR MUCH
MORE RARELY IN THEIR SUMMER DRESS, OR THEY RESEMBLE THE FEMALES ALONE.
OR THE YOUNG MAY HAVE AN INTERMEDIATE CHARACTER; OR, AGAIN, THEY MAY
DIFFER GREATLY FROM THE ADULTS IN BOTH THEIR SEASONAL PLUMAGES.
510