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, 1s much 
less marked in the lower than in the higher races. 
THE SKELETON OF THE FREE LIMBS 
As already indicated, the fore- and hind-hmbs 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 
