323 



at either one end or the other is to be considered progressive, and a 

 change in the opposite direction retrogressive. Rosenberg recognizes 

 that if these two opposed factors stood to one another in a fixed 

 relation it would be possible from the condition of one end of the 

 thoracic region to predicate that of the other, but he admits that, so 

 far from this being the case, the most various combinations occur. 

 In his first paper Rosenberg enunciated the principle of concomitant 

 variations, as for instance of the 20th vertebra frequently bearing a 

 rib when the 25th is free, which is undoubtedly true, though not in 

 all cases; for which it seems to me he has hardly received the credit 

 he deserved. He never, so far as I know, has given any explanation 

 of the cause of this phenomenon, beyond tacitly implying that when 

 the variation was in one direction it was to be accounted for by re- 

 version and in the other by progressive development. 



I hope to show that there is a principle to account for these 

 changes and that, looked at from another point of view, the con- 

 tradictory nature of the changes at the two ends of the thoracic region 

 which has just been alluded to, can in many cases be explained 

 Certainly a theory that shall do away with such difficulties has some- 

 thing in its favor. 



That there is much that is attractive in Rosenberg's theory is 

 proved by the enthusiasm with which it has for the most part been 

 received, which is natural when we consider 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 con- 

 tributions 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 by Holl, 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 embryo- 

 logical question, and shall therefore assume that Rosenberg's state- 

 ment is justified, and limit myself to discussing whether, if so, it 

 offers a satisfactory explanation of phenomena which it should ac- 

 count for. 



Irregular segmentation. Numerical variations are explained 

 also by intercalation and excalation, by which is meant the 

 presence of an additional vertebra, or the absence of a normal one, 



21* 



