194 



Until 1884 the general opinion among 

 those who speculated on phj'logenetic mat- 

 ters was that the group of mammals had 

 an amphibian ancestry. Huxley was es- 

 pecially prominent in advocating such a 

 line of descent, basing his conclusions upon 

 the naked, scaleless skin, the double oc- 

 cipital condyle and other cranial features, 

 some of which will be mentioned later. 

 Yet this view was not universal, since 

 Owen, in 1876, and Cope, in 1878 and in 

 later papers, had suggested a reptilian an- 

 cestry- for the group. Still, these specu- 

 lations attracted but little attention until 

 1884, when Caldwell's famous dispatch, 

 * Monotremes oviparous, eggs meroblastic,' 

 excited the enthusiasm of the British As- 

 sociation for the Advancement of Science, 

 then meeting in Montreal. At that date 

 embryology was the ruling force in deciding 

 questions of phylogeny, and the discovery 

 by Caldwell that the Echidna laid eggs, 

 and that these eggs were like those of the 

 reptiles rather than those of the amphibia 

 in their segmentation, at once suggested to 

 every zoologist a reptilian ancestry for the 

 mammals. This was still further empha- 

 sized a few days later by Cope's paper upon 

 the relations of the theriomorphous reptilia 

 and the monotreme mammalia, read before 

 the American Association at its Philadel- 

 phia meeting. 



Since that day numerous students have 

 built upon that foundation, and I need but 

 allude to the papers of Cope, Seeley, Osborn, 

 Howes, Lyddeker, Baur and Case, all of 

 which accept the reptiles as the progenitors 

 of the scaleless, hairy, milk-producing ver- 

 tebrates that we know as mammals. They 

 have brought forward much evidence — but 

 solely osteological in character — in support 

 of their views, and for the summary of this 

 which follows I am largely indebted to the 

 able papers of Osborn. 



The particular group of reptiles which 

 they have selected for this high honor is that 



SCIENCE. [N. s.y^oL. XIV. no. 345. 



known as Theriomorphs, the fossils of which 

 are found in rocks of Permian and Liassic 

 age in Illinois, Texas, New Mexico, Scot- 

 land, Bavaria, Bohemia, the Urals, Bengal 

 and South Africa. Then they suddenly 

 disappear, for no traces of them occur in 

 rocks of more recent age, and there is a vast 

 gap between them and the earliest mam- 

 mals of which we have any adequate knowl- 

 edge. This group shows several features in 

 which they approach the mammalia more 

 closely than do any other reptiles, and a 

 summary of these points may be of value 

 now. 



In the mammalia the skull is articulated 

 to the atlas, the first bone of the vertebral 

 column, by a pair of oval articular surfaces, 

 the occipital condyles. These are borne 

 one on each exoccipital bone. In most 

 reptiles, on the other hand, there is but a 

 single condyle, largely or wholly basioc- 

 cipital in origin. In many of these theri- 

 omorphs the exoccipitals partake in the 

 formation of this structure, and in some 

 the basioccipital portion exhibits a tend- 

 ency to recede, thus exhibiting a condition 

 which, carried still farther, would result in 

 two condyles like those of the mammals. 



In the mammals there is a heterodont 

 dentition ; that is, there are different kinds 

 of teeth — incisors, canines and molars. In 

 recent reptiles and in amphibia there is no 

 such differentiation, but in these therio- 

 morphs one group presents a dentition 

 which is strikingly suggestive of that of the 

 mammals. Indeed, one species, described 

 from a lower jaw, was at first regarded as 

 a mammal. 



In the mammals the anterior dorsal ribs 

 bear peculiar relations to the vertebrge. 

 These ribs bear two ' heads,' by which they 

 are articulated with the backbone. One of 

 these, the so-called tubercular head, articu- 

 lates with a process, the diapophysis, which 

 arises from the neural arch, while the other, 

 or capitular head, has its articulation with 



