to a particular season of the year, the sexual characters or organs are
developed at this season, and then disappear again, so that there is a
periodic development corresponding to the periodic activity of the testes
or ovaries. Stags have a limited breeding or 'rutting' season in autumn
(in north temperate regions), and the antlers also are shed and developed
annually. In this case we cannot assert that the development of the antler
takes place during the active state of the testes. The antlers are fully
developed and the velvet is shed at the commencement of the rutting
season, and development of the antlers takes place between the beginning
of the year and the month of August or September. In ducks and other birds
there is a brilliant male-breeding plumage in the breeding season which
disappears when breeding is over, so that the male becomes very similar to
the female. In the North American fresh-water crayfishes of the genus
Cambarus there are two forms of males, one of which has testes in
functional activity, while in the other these organs are small and
quiescent: the one form changes into the other when the testes pass from
the one condition to the other.

It has long been known that the development of male sex-characters is
profoundly affected by the operation of castration. The removal of the
testes is most easily carried out in Mammals, in consequence of the
external position of the organs in these animals, and the operation has
been practised on domesticated animals as well as on man himself from very
ancient times. The effect is the more or less complete suppression of the
male insignia, in man, for example, the beard fails to develop, the voice
does not undergo the usual change to lower pitch which takes place at
puberty, and the eunuch therefore has much resemblance to the boy or
woman. Many careful experimental researches have been made on the subject
in recent years. The consideration of the subject involves two questions:
(1) What are the exact effects of the removal of the gonads in male and