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the old idea of the archetype vertebra. There is no denying that the 

 costal elements by their greater or less development are the cause of 

 the immense majority of the variations. Originally this conception 

 implied a plan ; now it is tacitly admitted by all. The paragraph at 

 the head of this paper is the opening one of that masterpiece of de- 

 structive criticism, Herbert Spencer's review of Owen's Archetype 

 Skeleton. Certainly the extravagances of the dreamers who made 

 science ridiculous were easy prey; but though it was not hard to 

 dispose of any or all precise interpretations of the archetype skeleton, 

 the idea of plan remains. Spencer, after demolishing Owen, tried 

 as a substitute theory that of accumulated modifications; but it is far 

 from satisfactory. One may think Spencer had this in mind in the 

 first sentence of the paragraph above quoted. Indeed he points out 

 the shortcomings of his own theory with a rare frankness: "But it 

 may be replied, this hypothesis does not explain all the facts. It does 

 not tell us why a bone whose function in a given animal requires it 

 to be solid, is formed not of a single piece, but by the coalescence 

 of several pieces which in other creatures are separate: it does not 

 account for the frequent manifestations of unity of plan in defiance 

 of teleological requirements. This is quite true." The idea of plan 

 is not easy to get rid of. The mind of man craves it. If there be 

 nothing but absurdity in the idea of a type vertebra, how is it that 

 the actual thing, not imaginary amplifications of it, holds its own so 

 persistently? Let those who find a sufficient answer in "heredity" 

 tell us how the vertebral system became so securely intrenched from 

 the time of the first appearance of vertebrates that it has never been 

 dislodged. 



Beyond question, as was intimated above, one reason of the great 

 success of Rosenberg's theory has been that it fitted in so perfectly 

 with the doctrine of descent by gradual modifications. It has, unfor- 

 tunately for science, become too much the custom to make everything 

 square with this doctrine. If a certain occasional feature shows a 

 tendency opposite to the course of phylogenesis, it is too often inter- 

 preted as necessarily a step in the direction of future modifications. 

 There is at last some protest against the dogmatism that requires all 

 phenomena to be accounted for in accordance with a certain theory. 

 Thus Kohlbrugge: "I consider all so-called atavistic variations as 

 neutral in respect to the past or future type of the race, and occasioned 

 either by variation or by an arrest of development. This latter is 

 caused mostly by unknown accidental disturbances manifested by ir- 



