HUXLEY'S SKULL THEOllY. 295 



the anterior portion of the dorsal marrow. But the true 

 mode of empirically establishing this philosophic hypothesis 

 was yet to be discovered ; and this discovery we owe to 

 Gegenbaur.^^ He was the first to employ the phylogenetic 

 method, which, in this as in all morphological questions, 

 leads most surely and quickly to the result. He showed 

 that the Primitive Fishes (Selachii, Figs. 191, 192, p. 113), 

 as the parent-forms of all Amphirhina, yet retain per- 

 manently in their skull-structure that form of primordial 

 skull, from which the modified skull of the higher Verte- 

 brates, and therefore that of Man, has developed phylo- 

 geneticaUy. He also pointed out that the gill-arches of the 

 Selachii show that their primordial skull was originally 

 formed of a considerable number — at least nine or ten — 

 primitive vertebrae, and that the brain-nerves, which branch 

 from the base of the brain, entirely confirm this. These 

 brain-nerves — with the exception of the first and the second 

 pairs (the olfactory and the optic nerves) — are merely modi- 

 fied spinal nerves, and, in their peripheric distribution, 

 essentially resemble the latter. The Comparative Anatomy 

 of these brain-nerves is one of the strongest arguments for 

 the newer vertebral theory of the skulL 



It would lead us too far aside if we were to enter into 

 the particulars of this ingenious theory of Gegenbaur, and 

 I must content myself with referring to the great work 

 already quoted ; in it the theory is fully demonstrated by 

 empirical and philosophical arguments. The same author 

 has giveji a brief abstract in his " Outlines of Comparative 

 Anatomy " (1874), the study of which it is impossible to 

 recommend too highly. In this work Gegenbaur indi- 

 cates as original "skuU-ribs," or "lower arches of skull- 



