196 



SCIENCE. 



[N. S. Vol. XIV. No. 345. 



possible that they will bear another inter- 

 pretation than that usually accorded them. 



The argument from the heterodont den- 

 tition can have but moderate value, for we 

 know that different types of teeth have 

 been developed independently in different 

 groups of vertebrates, and it is possible 

 that they could have arisen in the mam- 

 mals and not have been inherited from the 

 theriomorphs. So, too, with the entepi- 

 condylar foramen. The nerve and blood 

 vessels exist in lower forms, and it is con- 

 ceivable that th6 existence of the foramen 

 in the two groups may be explained upon 

 the ground of homoplassy and without im- 

 plying inheritance. This foramen certainly 

 is of too little importance to be used as 

 the basis of great speculations. 



The matter of the ribs is more important. 

 In the amphibia with bicipital ribs, the 

 capitular head rests upon the so-called cen- 

 trum, and not between two centra, as in 

 theriomorphs and mammals. Yet, with our 

 present knowledge, this is far from conclu- 

 sive, for we know almost nothing of the 

 morphology of the vertebrse in most groups 

 of vertebrates. The researches of Fritsch 

 upon the fossil stegocephalan amphibia 

 have shown that the vertebral centra are 

 by no means simple affairs, but are really 

 composed of several (at least five) separate 

 elements. Traces of at least some of these, 

 more or less distinct, appear in 'the higher 

 vertebrates ; but until the homologies of 

 these are worked out for the existing am- 

 phibia, the reptiles and the mammals, argu- 

 ments based upon the relations of the ribs 

 to centra and intercentra must remain in- 

 conclusive. As it stands at present, it must 

 be admitted that the burden of proof, so far 

 as the ribs are concerned, is against the ad- 

 vocates of amphibian ancestry. 



The matter of the occipital condyles is 

 even less conclusive. Until the discovery 

 of the theriomorphs, the fact that both 

 amphibia and mammals have two condyles 



and the sauropsida but one condyle, was 

 regarded by Huxley as the very strongest 

 argument for amphibian ancestry, and the 

 most that is claimed for the double condyles 

 of the theriomorphs is that they show that 

 these animals are not to be counted out 

 upon reasons based upon the articulation 

 between cranium and vertebral column. 



Yet in none of these is there an exact 

 reproduction of the mammalian conditions, 

 for in all the basioccipital participates to a 

 greater or less extent in the formation of 

 the condyles, these structures being de- 

 scribed at times as distinctly bilobed, at 

 times having the basioccipital portion 

 receded below the level of the rest, but still 

 rather prominent. In other words, the 

 double condylar condition of the therio- 

 morphs — and hence that of the mammals — 

 is supposed to have arisen from the single 

 condyle of other forms by recession of the 

 basi-occipital. In the development of the 

 mammals there are, however, no traces of 

 such a stage. All this, however, is aside 

 from the more fundamental question. Is the 

 occipital region of the skull homologous 

 throughout the vertebrates ? 



Of far more importance than all these 

 features is the problem of the quadrate. 

 In fact, the whole matter of the ancestry of 

 the mammals may almost be said to hinge 

 upon the decision arrived at as to the fate 

 of the quadrate in the mammals. 



A brief review of some points of an 

 anatomical character may make clear the 

 discussion of the quadrate. In the first 

 place, it must be kept in mind — and this is 

 too frequently ignored by those who deal 

 with bones alone — that there are two kinds 

 of bones which differ greatl}' from each 

 other in history, both ontogenetic and phy- 

 logenetic, and that while one may seem- 

 ingly replace the other, there is no evidence 

 whatever of one passing into the other. 

 These two types are known respectively as 

 cartilage bones and membrane bones. A 



