1888.] 



On the Vertebral Chain of Birds. 



467 



On the Number of Vertebrce in Existing Birds. 



The Swan, one of the noblest of the Precocial birds, comes the 

 nearest of any of the Carinatse to the huge, almost wingless Struthious 

 types in the large number of its vertebrse ; indeed in the cervical 

 region it has more vertebras than any bird I have yet examined, 

 namely, twenty-five, and its general sacral region is as long as in the 

 large Ratitse, so that this bird, although so exquisitely specialised as a 

 flying, swimming, sailing, and walking bird, has not departed very 

 far from the. Struthious birds in respect of the length of the spine. 

 More than this, in my recent researches into the development of this 

 and cognate birds, I find that the Swan has been built upon the 

 Struthious foundation — so to speak. In the parts that suspend them- 

 selves from the twenty-one sacral vertebras, the hip-girdle moieties, 

 it is most clearly seen that the difference in these parts between this 

 bird and the African Ostrich (Struthio -camel us) is altogether one of 

 gentle transformation by a late growth of cartilage. Arrest the pelvis 

 of the embryo Swan, when only two- thirds ripe, and in the ossification 

 afterwards unite the pubes by ankylosis, and then the two pelves 

 would correspond, point by point. So that this part of the skeleton 

 passes in the most orderly manner, first through a general Reptilian, 

 then through an Ornithoscelidan, and then through a Struthious stage, 

 before it takes on the characteristic form of the Swan, the pelvis of 

 which is one of the largest and most remarkable in the class, and 

 quite typical, nevertheless, as the pelvis of a Carinate bird. 



Returning to the Vertebral Chain, I may now show how, for 

 adaptive purposes, that series of axial segments gets shorter and 

 shorter as we ascend towards the smallest and highest of the 

 " Altrices," the highest kind of birds, with tender young, and, as a 

 rule, arboreal nidification. 



Even within the limits of the Anatidas, the family to which the 

 Swan belongs, the cervical and sacral series get reduced to about 

 three-fourths the number found in the Common Swan. Indeed, in the 

 genus Cygnus, itself, I find a variation, for in C. nigricollis there are 

 only twenty-four cervical vertebras. 



But among the larger Precocial birds the number varies extremely, 

 and in passing from species to species, in the Cranes (Gruidce), I find no 

 two alike in this respect. Once, however, amongst the noblest and 

 most intelligent of all the birds, the Passerines, and we come upon a 

 uniformity that is as remarkable as the variety seen in the wading, 

 and land, and water birds. 



The Crows stand at the top of the Passerines, and being the largest 

 kind, they have the longest vertebral chain. 



In the old* Rook or Carrion Crow (Corvus frugilegus and 0. corone), 



* The fledgling is more generalised, and has twelve vertebra; united by the diapo- 

 physes in the sacral series. 



