56 THE DESCENT OP MAN. 



otlicr Primates, for characters, gained through sexual selection, 

 often differ to an extraordinary degree in closely-related forms. 



According to a popular impression, the absence of a tail Is 

 eminently distinctive of man; but as those apes which come near- 

 est to him are destitute of this organ, its disappearance does not 

 relate exclusively to man. The tail often differs remarkably in 

 length within the same genus: thus in some species of Macacus It 

 Is longer than the whole body, and is formed of twenty-four verte- 

 bras; in others it consists of a scarcely visible stump, containing 

 jnly three or four vertebras. In some kinds of baboons there are 

 twenty-five, whilst in the mandrill there are ten very small 

 stTinted caudal vertebrae or, according to Cuvier,™ sometimes only 

 five. The tail, whether it be long or short, almost always tapers 

 towards the end; and this, I presume, results from the atrophy of 

 the terminal muscles, together v/ith their arteries and nerves, 

 through disuse, leading to the atrophy of the terminal bones. But 

 no explanation can at present be given of the great diversity which 

 often occurs in its length. Here, however, we are more specially 

 concerned with the complete external disappearance of the tail. 

 Professor Broca has recently shown" that the tail in all quadru- 

 peds consists of two portions, generally separated abruptly from 

 each other; the basal portion consists of vertebra, more or less 

 perfectly channelled and furnished with apophyses like ordinary 

 vertebrae; whereas those of the terminal portion are not chan- 

 nelled, are almost smooth, and scarcely resemble true vertebras. 

 A tail, though not externally visible, is really present in man and 

 the anthropomorphous apes, and is constructed on exactly the 

 same pattern in both. In the terminal portion the vertebrae, con- 

 stituting the OS coccyx, are quite rudimentary, being much reduced 

 in size and number. In the basal portion, the vertebra are like- 

 wise few, are united firmly together, and are arrested in develop- 

 ment; but they have been rendered much broader and flatter than 

 the corresponding vertebrse in the tails of other animals: they 

 constitute what Broca calls the accessory sacral vertebrae. 

 These are of functional importance by supporting certain internal 

 parts and in other ways; and their modification is directly con- 

 nected with the erect or semi-erect attitude of man and the anthro- 

 pomorphous apes. This conclusion is the more trustworthy, as 

 Broca formerly held a different view, which he has now abandoned. 

 The modification, therefore, of the basal caudal vertebrae in man 

 and the higher apes may have been effected, directly or indirectly, 

 through natural selection. 



■» Mr. St. George Mivart, 'Proc. Zoolog. Soc, 1865, pp. 562, 5S3. Dr. J. K. 

 Gray, 'Cat. Brit. Mus. : Skeletons.' Owen, 'Anatomy of Vertebrates,' 

 vol. ii. p. 517. Isidore Geoffroy, 'Hist. Nat. Gen.' torn. li. p. 244. 



» 'Revue d'Anthropologle," 1872; 'La Constitution des Vertebres cau- 

 dales.' 



