282 THE EVOLUTION OF MAN. 



parent-form, from some Promammal which had seven neck- 

 vertebrae. If each animal species had been a distinct crea- 

 tion, it would have been far more to the purpose to have 

 furnished the long-necked Mammalia with a larger, and the 

 short-necked with a smaller number of neck-vertebras. The 

 neck- vertebrae are immediately folio wed by those of the breast 

 or thorax, which, in Man and most other Mammals, number 

 twelve or thirteen (usually twelve). Attached to the sides 

 of each breast- vertebra (Fig. 255) is a pair of ribs long 

 curved processes of bone lying in and supporting the wall of 

 the thorax. The twelve pairs of ribs, with the connecting 

 intercostal muscles and the breast-bone (sternum) constitute 

 the breast -body (thorax, Fig. 252, p. 279). In this elastic 

 and yet firm thorax lie the double lung, and between the 

 two halves of this, the heart. The chest-vertebrae are 

 followed by a short but massive section of the vertebral 

 column, formed by five large vertebras. These are the 

 lumbar- vertebras (Fig. 256), which bear no ribs and have 

 no perforations in their lateral processes. Next comes the 

 cross-bone (sacrum), which is inserted between the two 

 halves of the pelvic girdle. This cross-bone consists of five 

 fixed and amalgamated cross-vertebraa. Last comes a small 

 rudimentary tail- vertebral column, the rump-bone (coccyx). 

 This bone consists of a varying number (usually four, more 

 rarely three or five) of small aborted vertebra?; it is a 

 useless rudimentary organ, retaining no physiological sig- 

 nificance either in Man or in the Tail-less Apes or Anthro- 

 poids. (Of. Figs. 204-208.) Morphologically it is, however, 

 very interesting, as affording incontrovertible evidence of 

 the descent of Man and of Anthropoids from Long-tailed 

 Apes. For this assumption affords the only possible 



