76 THE STRUCTURE OF MAN 



The close connection between the great expansion of the 

 iliac bones and the upright gait of Man has already been pointed 

 out (ante, p. 38). 



The sexual dimorphism of the pelvis is more marked in Man- 

 kind than in any other Vertebrate ; indeed, it may be considered 

 as a characteristic of the human species, the rationale of which 

 has still to be discussed. 



If we consider the marked lateral projection of the iliac bones 

 which is met with in both sexes, and has already been described 

 and accounted for, it seems natural enough to regard their in- 

 creased expansion in the female as an adaptation to sexual re- 

 quirements. This increase of breadth is the more necessary, 

 since the human embryo attains a higher development before 

 birth than do the embryos of most Mammals, the skull and brain 

 being incomparably larger in proportion to the size of the mother. 

 So highly differentiated an embryo, again, must influence the pelvic 

 aperture, and, indeed, the whole form of the lower parts, includ- 

 ing the promontory, since the pressure of the pregnant uterus is 

 not exerted ventrally as in Quadrupeds, but, on account of the 

 upright gait, sagittally. The iliac wings thus play the chief part 

 in carrying this weight, and naturally undergo a corresponding 

 lateral plate-like expansion. Further investigation concerning the 

 pelvis in relation to " labour " in the different races of Mankind 

 would be of great interest. All that can now be stated with 

 certainty is, that sexual differentiation of the pelvis, at least so 

 far as the expansion of the iliac bones is concerned, is much 

 less marked in the lower than in the higher races. 



THE SKELETON OF THE FREE LIMBS 



As already indicated, the fore- and hind-limbs of Man con- 

 form to a single type ; and any doubt which might exist as to the 

 differences between the two having been secondarily acquired by 

 functional adaptation, is dispelled by Comparative Anatomy and 

 Ontogeny. As already pointed out (pp. 68, 69), a review of the 

 various groups of Vertebrata shows that the farther we go back 

 in the series the less marked are the differences between the fore- 

 and hind-limbs ; until at length, in the Fishes, we have an undif- 

 ferentiated starting-point for the two. At the top of the scale 

 we have the Birds with their fore-limbs metamorphosed into wings 

 (under conditions by which the pelvis and vertebral column 

 become correlatively modified with the hind-limbs, to support the 



