1888.] possibly Dual Origin of the Mammalia. 



375 



earliest mammals which first had teeth of the mammalian type, or 

 have arisen independently. 



Let us first briefly consider the former alternative ; such a mam- 

 malian ancestor must, on the generally received doctrine of evolution, 

 have had its general organisation like that of an existing Monotreme, or 

 have been formed on a yet lower type. In either case if all mammals 

 furnished with grinding teeth have also proceeded from such early 

 root form, it is remarkable that none of its descendants save the 

 Monotremes have inherited those skeletal, cerebral and genito-urinary 

 peculiarities which characterise the Ornithodelphia, and which, on 

 this hypothesis, must also have been possessed by the various 

 ancestors of the different orders of non-monotrematous mammals. In 

 that case, the creatures which came to form all these orders must 

 have simultaneously and persistently varied in a single direction, 

 resulting in that one very definite form of organisation which is com- 

 mon to the placental and marsupial mammals. But this will 

 probably be considered an all but utterly inadmissible supposition. 



If, however, the Ornithorhynchus tooth arose in some much less 

 primitive mammal, one which was previously edentulous or had but 

 Sauropsidan teeth, and therefore was not also the progenitor of all the 

 other mammals with grinding teeth, then such teeth must have twice 

 arisen independently, and there seems, on this view, no reason to 

 repudiate the other alternative, namely, that the Ornithorhynchus 

 teeth might have arisen independently, in relatively modern times, in 

 what may have been no very remote ancestor of the Ornithorhynchus 

 itself. In that case, however, the wonder remains that the Mono- 

 tremes should have retained so many Sauropsida-like features which 

 all other mammals have entirely lost. 



The question then presents itself, is it possible that the Mono- 

 tremes may be instances of degradation ; that they inherit their teeth 

 from early but ordinary toothed mammals, while their shoulder- 

 structure, rudimentary corpus callosum, and genito-urinary peculiar- 

 ities are due to degradation and reversion ? It is now considered 

 by some naturalists that the Amphioxus and the Tunicates are 

 extremely degraded Vertebrates. 



When we recall to mind such instances amongst the Invertebrata as 

 Lerneocera and Sacculina, any amount of degradation seems possible. 

 As to the corpus callosum, considerable differences exist amongst the 

 Placentalia, and it is difficult to see why it might not sometimes 

 shrink as well as augment, and we must admit that the optic chiasma 

 has disappeared in Teleostean fishes, if they had, as would be generally 

 admitted, either Ganoid-like or Elasmobranch-like ancestors. A cloaca 

 is absent in mammals which are not Monotremes, yet such a structure, 

 though very shallow, has reappeared in Kodents and Edentates 

 (Beaver and Sloth). The penis is strangely modified, but the pro- 



