female. These might be called the organs of courtship, such as the
peacock's tail, the plumes of the birds-of-paradise, and the brilliant
plumage of humming birds and many others. The song of birds is another
example, and sound is produced in many Fishes for a similar purpose.

5. ORGANS FOR THE BENEFIT OF THE OFFSPRING: for example, the extraordinary
pouches in which the eggs are developed in certain Frogs. In the South
American species, _Rhinoderma darwinii_, the enlarged vocal sacs are used
for this purpose. Pouches with the same function are developed in many
animals, for instance in Pipe-fishes and Marsupials. Abdominal appendages
are enlarged in female Crustacea for the attachment of the eggs, the
abdomen also being larger and broader.

The argument in favour of the Lamarckian explanation of the evolution of
these adaptive characters is the same as in the case of adaptations common
to both sexes, namely that in every case the function of the organs and
characters involves special irritations or stimulations by external
physical agents. Mechanical irritation, especially of the interrupted
kind, repeated blows or friction causes hypertrophy of the epidermis and
of superficial bone. I have stated this argument and the evidence for it
in some detail in my volume on _Sexual Dimorphism_. It is one of the most
striking facts in support of this argument that the hypertrophied plumage
which constitutes the somatic sexual character of the male in so many
birds is habitually erected by muscular action for the purpose of display
in the sexual excitement of courtship. I doubt if there is a single
instance in which the male bird takes up a position to present his
ornamental plumage to the sight of the female without a special erection
and movement of the feathers themselves. Such a stimulation must affect
the living epidermic cells of the feather papilla. Even supposing that the
feather is not growing at the time, it is probable, if not certain, that