ON THE THEORY OF THE VERTEBRATE SKULL 541 



a long series of the skulls and vertebral columns of adult Vertebrata,. 

 determining, in this way, the corresponding parts of those which are 

 most widely dissimilar, by the interpolation of transitional gradations 

 of structure. Using the other method, the investigator traces back 

 skull and vertebral column to their earliest embryonic states, and 

 determines the identity of parts by their developmental relations. 



It were unwise to exalt either of these methods at the expense of 

 its fellow, or to be other than thankful that more roads than one 

 leads us to the attainment of truth. Each, it must be borne in mind, 

 has its especial value and its particular applicability, though at the 

 same time it should not be forgotten, that to one, and to one only,, 

 can the ultimate appeal be made, in the discussion of morphological 

 questions. For seeing that living organisms not only are, but become, 

 and that all their parts pass through a series of states before they 

 reach their adult condition, it necessarily follows that it is impossible- 

 to say, that two parts are homologous or have the same morphological' 

 relations to the rest of the organism, unless we know, not only that 

 there is no essential difference in these relations in the adult con- 

 dition, biit that there is no essential difference in the course by 

 which they arrive at that condition. The study of the gradations of 

 structure presented by a series of living beings may have the utmost 

 value in suggesting homologies, but the study of development alone 

 can finally demonstrate them. 



Before the year 1837, the philosophers who were occupied with 

 the Theory of the Skull confined themselves, almost wholly, to the 

 first-mentioned mode of investigation, which may be termed the 

 " method of gradations." If they made use of the second method 

 at all, they went no further than the tracing of the process of ossifi- 

 cation, which is but a small, and by no means the most important 

 part of the whole series of developmental phenomena, presented by 

 either the skull or the vertebral column. 



But between the years 1836 and 1839, the appearance of three 

 or four remarkable Essays, by Reichert, Hallmann, and Rathke.i 

 inaugurated a new epoch in the history of the Theory of the Skull. 

 Hallmann's work on the Temporal Bone is especially remarkable for 

 the mass of facts which it contains, and for that clearness of insight 



^ The titles of these works are, — Reichert, ' De Embryonum arcubus sic dictis Branchia- 

 libus,' 1836, which I have not seen ; the same writer's essay, ' Ueber die Visceralbogen der 

 Wirbelthiere im Allgemeinen,' Muller's Archiv, 1837. Hallmann, ' Die vergleichende 

 Osteologie des Schlafenbeins,' 1837. Rathke, ' Entwickelungsgeschichte der Natter,' 1839. 

 I regret that, in spite of all efforts, I have hitherto been unable to procure a copy of another 

 very important work of Rathke's, the ' Programm,' contained in the ' Vierter Bericht von 

 dem naturwissenschaftlichen Seminar zu Konigsberg.' 



