76 GENERAL ANATOMICAL CHARACTERS 



into the mouth of the young animal by the action of a compressor 

 muscle. 



Secondary Sexual 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 Sex, 

 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- 

 fogoii), 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 Ornithorhynchns 

 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 groAvth, 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, 



