150 THE DESCENT OF MAN. PAST I- 
surprising that man should differ so greatly in hairi- 
ness from all his lower brethren, for characters gained 
through sexual selection often differ in closely-related 
forms to an extraordinary degree. 
According to a popular impression, the absence of a 
tail is eminently distinctive of man ; but as those apes 
which come nearest to man are destitute of this organ, 
its disappearance does not especially concern us. Never- 
theless it may be well to own that no explanation, as 
tar as I am aware, has ever been given of the loss of the 
tail by certain apes and man. Its loss, however, is not 
surprising, for it sometimes differs remarkably in length 
in species of the same genera: thus in some species 
of Macacus the tail is longer than the whole body, con- 
sisting of twenty-four vertebras ; in others it consists of a 
scarcely visible stump, containing only three or four 
vertebrae. In some kinds of baboons there are twenty- 
five, whilst in the mandrill there are ten very small 
stunted caudal vertebras, or, according to Cuvier , 79 some- 
times only five. This great diversity in the structure and 
length of the tail in animals belonging to the same genera, 
and following nearly the same habits of life, renders it 
probable that the tail is not of much importance to 
them ; and if so, we might have expected that it would 
sometimes have become more or less rudimentary, 111 
accordance with what we incessantly see with other struc- 
tures. 'The tail almost always tapers towards the end 
whether it be long or short ; and this, I presume, re- 
sults from the atrophy, through disuse, of the terminal 
muscles together with their arteries and nerves, lead- 
ing to the atrophy of the terminal bones. With respect 
79 Mr. St. George Mivart, ‘Proc. Zoolog. Soc.’ 1SG5, p. 562, 583- 
Dr. J. E. Gray, ‘Cat. Brit. Mus. : Skeletons.’ Owen, ‘Anatomy 0 
Yertebrates,’ vol. ii. p. 517. Isidore Geoffroy, ‘Hist. Nat. Ge'n.’ tom. 
li p. 244. 
