372 



Prof. St. G. Mivart. On the 



[Feb. 16, 



IV. "On the possibly Dual Origin of the Mammalia." By St. 

 George Mivart, M.D., F.R.S. Received February 14, 1888. 



The recent discovery by Mr. Edward B. Poulton of non-functional 

 teeth hidden beneath the bony plates of the jaws of the young Orni- 

 thorhynchus is not only most interesting in itself, but taken in con- 

 nexion with another recent discovery as to the anatomy of that 

 animal, exceedingly suggestive. It is, of course, easy to assign too 

 great a value to the forms of teeth, and everyone knows how Cuvier 

 was thus led to associate the marsupial Carnivora with the placental 

 Carnivores. There is an evident temptation also to exaggerate the 

 significance of dental structure, both on account of the obvious nature 

 of such characters and also because they are so exceptionally well 

 preserved in fossil remains. But no zoologist can deny that the 

 value of dental characters is often exceedingly great, and when, as in 

 the case of Ornithorhynchus, we have them in the form of living 

 fossils, as it were, entombed within the jaws, we may fairly presume 

 that they show us what their shape was when they were last in 

 actual use, and so must possess a greater or less taxonomic value. The 

 most valuable evidences of affinity are commonly afforded by struc- 

 tures less distinctly related to habits of life. Thus, for example, the 

 course taken by the internal carotid artery has often a more profound 

 significance than has either the structure of the teeth or sha*pe of the 

 limbs ; while the possession by any two animals of a prehensile tail 

 — in spite of the niceties of structure which concur to produce it — 

 cannot alone be accepted as a test that they belong even to the same 

 order. 



The shape of the teeth, having a manifest direct relation to condi- 

 tions of life, requires, then, a very careful criticism before any 

 evidence it may seem to afford can be relied on as a test of affinity. 



The Ornithodelphia (Ornithorhynclius and Echidna) have long been 

 known to possess characters resembling the Sauropsida and especially 

 the Lacertilian Reptilia. Nevertheless, no less distinguished an 

 anatomist than Professor Huxley has, so late as 1880, regarded them 

 as descendants (through imaginary creatures called Hypotheria) from 

 amphibians and not from any of the Sauropsida ;* a view which I 

 myself have also held. 



The most interesting discovery by Mr. Caldwell of the eggs of Orni- 

 fhodelphia, the announcement of which startled the meeting of the 

 British Association in Canada, greatly strengthened the evidence pre- 

 viously relied on by certain naturalists, that the Ornithodelphia 

 descended from some Reptilian form, and this view seems to have 



* ' Zool. Soc. Proc.,' 1880, p. 662. 



