THE SHAPES OP VERTEBRATE SKELETONS. 223 



mechanical actions and reactions must progress from without 

 inwards, affecting last of all the axis; and if, as we have 

 seen, the region of the head is so circumstanced that the 

 causes of segmentation act but feebly even on its periphery; 

 then that terminal portion of the primitive notochord which 

 is included in the head, having to undergo no lateral land- 

 ings, may ossify without division into segments. 



Of other incidental evidences supplied by comparative 

 morphology, let me next refer to the supernumerary bones, 

 which the theory of Goethe and Oken as elaborated by Prof. 

 Owen, has to get rid of by gratuitous suppositions. In many 

 fishes, for example, there are what have been called inter- 

 neural spines and inter-haemal spines. These cannot by any 

 ingenuity be affiliated upon the archetypal vertebra, and 

 they are therefore arbitrarily rejected as bones belonging to 

 the exo-skeleton ; though in shape and texture they are 

 similar to the spines between which they are placed. On the 

 hypothesis of evolution, however, these additional bones are 

 accounted for as arising under actions like those that gave 

 origin to the bones adjacent to them. And similarly with 

 such bones as those called sesamoid; together with others 

 too numerous to name. 



§ 259. Of course the foregoing synthesis is to be taken 

 simply as an adumbration of the process by which the verte- 

 brate structure may have arisen through the continued 

 actions of known agencies. The motive for attempting it has 

 been two-fold. Having, as before said, given reasons for con- 

 cluding that the segments of a vertebrate animal are not 

 homologous in the same sense as are those of an annulose 

 animal, it seemed needful to do something towards showing 

 how they are otherwise to be accounted for ; and having here, 

 for our general subject, the likenesses and differences among 

 the parts of organisms, as determined by incident forces, it 

 seemed out of the question to pass by the problem presented 

 by the vertebrate skeleton. 



