Mr. T. H. Huxley on the Theory of the Vertebrate Skull. 415 



As there are two problems, so there are two methods of obtaining 

 their solution. Employing the one, the observer compares together 

 a long series of the skulls and vertebral columns of adult Fertebrata, 

 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 

 lead 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 idtimate 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, but 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*, 

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

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

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

 into the architecture of the skull, which enabled him to determine the 

 homologies of some of the most important bones of its upper arch 

 throughout the vertebral series. Rathke showed the singular nature 

 of the primordial cranial axis ; and Reichert pointed out in what way 

 alone the character of its lower arches could be determined. For the 



* The titles of these works are, — Reichert, ' De Erabryonum arcubus sic dictis 

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

 Visceralbogen derWirhelthiere im Allgemeinen,' Midler's Archiv,1837. Hallmann, 

 ' Die vergleichende Osteologie des Schlafenbeins,' 1837. Rathke, * Entwicke- 

 lungsgeschichte 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 natur- 

 wissenschaftlichen Seminar zu Konigsberg." 



