26 A BOOK OF 'WHALES 



not from three, as might have been the case were 

 it the equivalent of the three bones ilium, ischium, 

 and pubis, which constitute the normal mammalian 

 pelvis. Professor Delage has ingeniously argued 

 in favour of the theory that the single bone of the 

 Cetacea represents the entire series in the ordinary 

 mammals.* 



The continuity of the partly bony, partly carti- 

 laginous mass is not necessarily fatal to the view ; 

 for where there are three separate bones (not to 

 mention the small cotyloid) the cartilage which they 

 replace is at first a perfectly continuous mass ; and 

 as to the appearance of but one centre of ossification 

 in this mass which gradually invades the whole, or 

 nearly the whole, it may be that prolonged investiga- 

 tions will show that there are other ossifications ; and 

 in any case it might be that the whole mass being so 

 reduced had only room, so to speak, for one centre 

 of ossification. In any case there is a considerable 

 superficial similarity between the small pelvis of 

 Balcenoptera and the fully developed pelvis of other 

 mammals ; there is a forward extension suggestive of 

 an ilium, a downward process which might do duty 

 for a pubis, and a hollow in the middle of the bone 

 which is not at all unlike the glenoid cavity ; in this, 

 indeed, the rudimentary femur is lodged. The 

 question is interesting as a general example of what 

 happens when reduction through degeneration takes 

 place. 



We shall recur to it presently, and in the meantime 



* Arc/i. de Zoolog. Experimental, 1887. 



