32 THE MAMMALIA, 



entirely changed mode of life of the Cetacea has 

 not ohscured, or only superficially obscured, these 

 characteristics ; for even the loss of the hind limbs 

 is of subordinate importance compared with other 

 significant peculiarities of the class, and moreover 

 the loss of limbs is met with in other classes, and 

 only shows to what extent the members of one 

 group may diverge. 



We will mention one characteristic in the 

 skeleton which distinguishes the whole group of 

 existing mammals from all other vertebrates : their 

 lower jaw is directly attached to the skull, and 

 not by means of the so-called * quadrate bone.' 

 This bone is met with from the fish to the bird, 

 mostly as one of considerable size. Indications 

 of it occur also among mammals. The substance, 

 however, does not harden into bone and become' 

 the stalk of the lower jaw so easily recognised 

 in the heads of birds, but is employed in forming 

 one of the small bones of the ear. Further, in all 

 of the Mammalia the chest and abdomen are sepa- 

 rated by the diaphragm or midriff, a muscle which 

 is exceedingly important for the mechanism of 

 breathing. All the Mammalia have lacteal glands, 

 and in the case of most mammals the foetus is 

 attached to the mother by a placenta, so that the 



