76 GENERAL ANATOMICAL CHARACTERS 
into the mouth of the young animal by the action of a compressor 
muscle. 
Secondary Seawal Characters—Secondary sexual characters, or 
modifications of structure peculiar to one sex, but not directly 
related to the reproductive function, are very general in mammals. 
They almost always consist of the acquisition or perfection of some 
character by the male as it attains maturity, which is not found in 
the female or the young in either sex. In a large number of cases 
these clearly relate to the combats in which the males of many 
species engage for the possession of the females during the breeding 
season; others are apparently ornamental, and of many it is still 
difficult to apprehend the meaning. Many suggestions on this 
subject will, however, be found in the chapters devoted to it in 
Darwin’s work on The Descent of Man and Selection in Relation to Ses, 
where most of the best-known instances are collected. Superiority 
of size and strength in the male of many species is a well- 
marked secondary sexual character related to the purpose indicated 
above, being probably perpetuated by the survivors or victors in 
combats transmitting to their descendants those qualities which 
gave them advantages over others of their kind. To the same 
category belong the great development of the canine teeth of the 
males of many species which do not use these organs in procuring 
their food, as the Apes, Swine, Musk and some other Deer, the tusk 
of the male Narwhal, the antlers of Deer, which are present in most 
cases only in the males, and the usual superiority in size and 
strength of the horns of the Bovidw. Other secondary sexual 
characters, the use of which is not so obvious, or which may only 
relate to ornament, are the presence of masses or tufts of long hair 
on different parts of the body, as the mane of the male Lion and 
Bison, the beards of some Ruminants and Bats (as Taphozous melano- 
pogon), Monkeys, and of Man, and all the variations of coloration 
in the sexes, in which, as a general rule, the adult male is darker 
and more vividly coloured than the female. Here may also be 
mentioned the presence or the greater development of odoriferous 
glands in the male, as in the Musk Deer, and the remarkable 
perforated spur with its glands and duct, so like the poison-tooth 
of the venomous serpents, found in the males of both Ornithorhynchus 
and Echidna, the use of which is at present unknown. 
Placenta.—The development of the mammalian ovum, and the 
changes which the various tissues and organs of the body undergo 
in the process of growth, are too intricate subjects to be explained 
without entering into details incompatible with the limits of this 
work, especially as they scarcely differ, excepting in their later 
stages, from those of other vertebrates, upon which, owing to the 
greater facilities these present for examination and study, the 
subject has been more fully worked out. There are, however, 
