Professor 0. C. Marsh — Origin of Mammals. 15 



skull by a distinct articulation. All reptiles, even those supposed to 

 be nearest the mammals, have the lower jaw composed of several 

 pieces, and these show distinct sutures between them, a profound 

 difference that must be explained away before an approach can be 

 made between the two classes. 



It may fairly be said that the separate elements of the lower jaw, 

 if present, would naturally be looked for in the Mesozoic mammals, 

 and this point I have long had in mind. I may safely say that 

 I have seen nearly every species of Mesozoic mammal hitherto 

 described, and have searched for evidence on this point without 

 success. I have also sought for the separate elements in the young 

 of recent forms, but without finding any indications of them. 



Besides the crucial points I have mentioned in the skull, there 

 are others of equal importance in the skeleton, which I must not 

 take time to discuss, but will venture to allude to one of them in 

 passing. I refer to the ankle joint, which, when present, is at the 

 end of the tibia in mammals, and in reptiles between the first and 

 second series of tarsals. When we really find an approach between 

 these two classes, the ankle joint will probably show evidence of it. 



Having thus shown, as I believe, that we cannot with our present 

 knowledge expect to find the origin of mammals among the known 

 extinct reptiles, and that in attempting this we are probably off 

 the true line of descent, it remains to indicate another direction in 

 which the quest seems more promising. 



Since 1876, when Huxley visited me at New Haven, and we 

 discussed the probable origin of both Birds and Mammals, I have 

 been greatly impressed by his suggestion that the mammals were 

 derived from ancestors with two occipital condyles, and these were 

 doubtless primitive Amphibians. I have since sought diligently for 

 the ancestors of birds among the early reptiles, with, I trust, some 

 measure of success, but this is a simple problem compared with the 

 origin of mammals which we have before us to-day. 



In various interviews with Francis Balfour in 1881, at the York 

 Meeting of the British Association, we discussed the same questions, 

 and agreed that the solution could best be reached by the aid of 

 Embryology and Palasontology combined. He offered to take up 

 the young stages of recent forms, and I agreed to study the fossils 

 for other evidence. His untimely death, which occurred soon after- 

 wards, prevented this promised investigation, and natural science still 

 suffers from his loss. Had Balfour lived, he might have given us 

 to-day the solution of the great question before us, and the present 

 discussion would have been unnecessary. 



The Birds, like the mammals, have developed certain characters 

 higher than those of reptiles, and thus they seem to approach each 

 other. I doubt, however, if the two classes are connected genetically, 

 unless in a very remote way. 



Eeptiles, although much lower in rank than birds, resemble 

 mammals in various ways, but this may be only an adaptive 

 likeness. Both of these classes may be made up of complex groups 

 only distantly related. Having both developed along similar lines, 



