529 



Is an important point ; and it is one on which Professor Owen mom 

 fe.4ly places great dependence. In his discussion of " moot cases 

 of special homology," it is the general test to which he appeals. 

 The typical natures of the alisphenoid, the mastoid, the orbito- 

 spheiioid, the prefrontal, the malar, the squamosal, &c., he deter- 

 mines almost wholly by reference to the adjacent nerve-perforations 

 and the articulations with neighbouring bones (see pp. 19 to 72) : 

 the general form of the argument being This bone is to be classed 

 as such or such, because it is connected thus and thus with these 

 others, which are so and so. Moreover, by putting forth an " ideal 

 typical vertebra," consisting of a number of elements standing 

 towards each other in certain definite arrangement, this persistency 

 of relative position is manifestly alleged. The essential attribute 

 of this group of bones, considered as a typical group, is the con- 

 Btancy in the connexions of its parts : change the connexions, and 

 the type is changed. But the constancy of relative position thus 

 tacitly asserted, and appealed to as a conclusive test in " moot 

 cases of special homology," is clearly negatived by Professor 

 Owen's own facts. For instance, in the " ideal typical vertebra," 

 the haemal arch is represented as formed by the two haemapophyses 

 and the haemal spine; but at p. 91 we are told that 



" The contracted haemal arch in the caudal region of the body may be 

 formed by different elements of the typical vertebra : e.g., by the para- 

 pephyses (fishes generally) ; by the pleurapophyses (lepidosiren) ; by both 

 pyrapophyses and pleurapophyses (Sudis, Lepldosteas), and by haemapo- 

 physes, shortened and directly articulated with the centrums (reptiles and 

 mammals)." 



And further, in the thorax of reptiles, birds, and mammals, " the 

 hsemapophyses are removed from the centrum, and are articulated to 

 the distal ends of the pleurapophyses ; the bony hoop being com- 

 pleted by the intercalation of the haemal spine " (p. 82). So that 

 there &refive different ways in which the haemal arch may be formed 

 four modes of attachment of the parts different from that shown 

 in the typical diagram ! Nor is this all. The pleurapophyses " may 

 be quite detached from their proper segment, and suspended to the 

 haemal arch of another vertebra ;" as we have already seen, the 

 entire haemal arch may be detached and removed to a distance, 

 sometimes reaching the length of twenty-seven vertebrae ; and, even 

 more remarkable, the ventral fins of some fishes, which theoretically 

 belong to the pelvic arch, are so much advanced forward as to bo 

 articulated to the scapular arch " the ischium elongating to join 

 the coracoid." With these admissions it seems to us that relative 

 position and connexions cannot be appealed to as tests of homology, 

 nor as evidence of any original type of vertebra. 



In no class of facts, then, do we find a good foundation for the 

 hypothesis of an " ideal typical vertebra." There is no one con- 



