August 9, 1901.] 



SCIENCE. 



201 



it as a labial — and this has apparently been 

 concerned in the transferrence of the hinge 

 of the lower jaw. This, however, is men- 

 tioned only as a suggestion ; details have 

 yet to be worked out, and further study 

 may show that this view is untenable. 



Now, summing up the evidence of the 

 bones, we may say that the characteristics 

 of the ribs, the heterodont dentition, and 

 the entepicondylar foramen point more 

 strongly towards a reptilian rather than to- 

 wards an amphibian ancestry for the mam- 

 mals. On the other hand, the occipital 

 condyles are even stronger evidence in the 

 other direction. But, when we consider 

 the relations of the ear bones and the quad- 

 rate, the weight of argument is very 

 strongly opposed to a reptilian ancestry, 

 while these same relations, and especially 

 the articulation of the quadrate with the 

 stapes, go far towards supporting the the- 

 ory that the mammals have descended from 

 the amphibia. 



There is another series of osteological 

 facts which also seems to point in the same 

 direction. In the mammals, as in the 

 amphibia, the ankle joint is formed between 

 the bones of the shank (tibia and fibula) 

 and the proximal row of tarsal bones. In 

 all reptiles of which we have adequate 

 knowledge, the joint is between the proxi- 

 mal and distal rows of tarsal bones — is 

 intratarsal. This, however, is not conclu- 

 sive, since the foot structure of the therio- 

 morphs is very imperfectly known. 



Besides the osteological evidence for the 

 descent of the mammalia from amphibian- 

 like forms, there are facts derived from the 

 soft parts which have a cumulative value. 

 They, however, are not conclusive, for we 

 cannot say what may have been the re- 

 lations in the theriomorphs. It may be 

 that these extinct reptiles possessed one or 

 all of these features, but the fact that they 

 are lacking in all modern reptiles lends 

 plausibility to the view that they were ab- 



sent from the older members of the group. 

 A detailed account of these would far 

 transcend the limits of this paper, and 

 but the briefest mention can be made of 

 them. 



In the first place, mammals are strongly 

 marked off from all other vertebrates by 

 the existence of hair. For a long time it 

 was thought that hair, feathers and scales 

 were homologous structures, but Maurer 

 has shown that hair is totally different from 

 the others. It is true that Weber has criti- 

 cised Maurer, but his criticisms seem far 

 from conclusive. According to Maurer — 

 and he offers a large mass of facts in sup- 

 port of his contention — the only structures 

 in the lower vertebrates which can be con- 

 ceived to have given rise to hair are lateral - 

 line organs of the amphibian type. Now, 

 lateral-line organs are unknown in any 

 terrestrial form. Even in the frogs and 

 salamanders they are lost during the meta- 

 morphosis which precedes a life on land. 

 Hence it is very probable that they were 

 lacking in the theriomorphs, all of which 

 were apparently terrestrial in habit. 



The student should also read Klaatsch's 

 account of the mesenterial structures, es- 

 pecially of the superior mesenteric artery, 

 to see how impossible it is to derive the 

 relations of these from any known con- 

 dition in the reptiles. In the mammals 

 these structures are far more primitive than 

 in the reptiles, and Klaatsch concludes that 

 their origins must be sought in forms below 

 the existing amphibia. 



Mammals alone have well-developed ex- 

 ternal ears, and these, as well as the tube 

 leading to the drum — the external meatus — 

 are supported by cartilages. All who have 

 worked at these agree that they must have 

 been derived from opercular structures, like 

 those of fishes, supported by the hyoid arch . 

 Now, all such structures are absent from all 

 known reptiles, nor do we know of them in 

 the amphibia. They must be sought in 



