NUMERICAL VARIATION IN THE HUMAN SPINE. 239 



of the thoracic region which has just been aUuded to, can in man}^ cases be explained. 

 Certainly a theory that shall do away with such difficulties has something in its favor. 



That there is much that is attractive in Rosenberg's theory is proved by the enthusi- 

 asm with which it has for the most part been received, which is natural when we con- 

 sider how perfectly' in many ways it fits in with the prevailing theory of evolution by 

 gradual changes. He has the satisfaction of knowing that practically all contributions to 

 this subject resolve themselves into a discussion of the validity of his views. 



It is remarkable, however, that while Rosenberg's theory is so very generally 

 accepted, and yet so strongly opposed by a few, his statement that the ilium enters into 

 relation with different vertebrae during the development of the individual human foetus 

 has never been confirmed ; while it has been contradicted bj^ Holl ('82) . who holds that the 

 vertebra which is the 1st sacral in the adult is such from the beginning. I myself have 

 not had the chance to work on this embryological question, and shall therefore assume 

 that Rosenberg's statement is justified, and limit myself to discussing Avhether, if so, it 

 offers a satisfactory explanation of phenomena which it should account for. 



Irregular Segmentation. — Numerical variations are explained also l)y intercalation 

 and excalation, by which is meant the presence of an additional vertebra, or the absence of 

 a normal one, between two particular vertebrae. There is some difference in the practical 

 use of this method, some authorities meaning that in the latter case a certain vertebi'a is 

 wanting and (perhaps) in the former that a certain vertebra is duplicated ; while others do 

 not go into such detail. If no particular vertebra be referred to, this theory is not very 

 different from that of irregular segmentation. There certainly are cases that can be 

 accounted for only by one of these methods. The best examples are found in the lower 

 forms. The following observations by Baur ('97) are very convincing. 



All hving crocodiles have 24 praesacral vertebrae, but in a certain gavial he found 25. 

 There is usually a sudden change in the transverse process of the 12th which is the first to 

 have the heads of the ribs on the transverse processes only. In this gavial the change 

 occurred on the 13th. The 9th and 10th of the normal spine have the capitellum of the 

 rib resting on the centrum just at the level of the neuro-central suture. In this gavial 

 there were three such vertebrae. Hence Baur concludes that there is an intercalation 

 between the 9th and 10th. It would be better, in my opinion, to say that in the place of 

 two vertebrae there are three, and not to attempt any more precise homology. I agree with 

 Bateson ('94) that " individuality should not be attributed to a member of a series which 

 has normally a definite number of members." Mention should here be made of the cele- 

 brated python in the Museum of the Royal College of Surgeons, discussed both by Baur 

 ('91) and Bateson, in which the 166th and the 185th vertebrae are double on one side, 



