96 H. F. shorn — Origin of Mammals, 



The first question is now unanswerable, although at present 

 the evidence that the Mammalia are diphyletic is certainly 

 considerable. Second, the investigation of the placental 

 ovum rises to extreme importance; if the Eutherian ovum is of 

 Amphibian type, the Mammalia are certainly diphyletic, for 

 the Monotreme ovum is certainly of reptilian type. This 

 point may be met by supposing that at the time the 

 Marsupio-placentals were given off, the Theriodontia conserved 

 a number of Amphibian characters, which were at the time or 

 subsequently lost. 



The problem of most immediate concern, therefore, is 

 whether the Theriodontia are actually the long-sought Pro- 

 mammalia of Hseckel, Hypotheria of Huxley or Sauromam- 

 malia of Baur, or whether they present a fresh instance of 

 extensive parallelism due to the assumption of habits analogous 

 to those of Mammalia. Professor Seeley has just presented 

 the latter view,* and it is certainly true that none of the 

 known Theriodontia fill the characters outlined above as those 

 we must look for in the Promammal or Eutherian stem, for 

 they are all too large and too specialized. 



There are however grounds for the more sanguine former 

 view, that the Theriodontia are the Hypotheria or Promam- 

 malia, because it appears that within the order may well have 

 existed some small insectivorous types, far less specialized in 

 tooth structure than either the carnivorous Cynodonts or her- 

 oivorous Gomphodonts, as one of those conservative spurs of 

 adaptive radiation which form the focus of a new progressive 

 type. 



The problem will therefore be settled by additional discovery 

 and knowledge of the structure of the diverse types which 

 undoubtedly composed this remarkable group. 



Note. We can happily preserve that part of Huxley's speculation upon the 

 origin of the Mammals which pictured the Insectivora as nearest the ancestral 

 type. The Amphibian (or Stegocephalian) origin of the Mammalia which he 

 defended, is wholly set aside, if the view here taken is correct, for the Theriodontia 

 are certainly not Amphibia in any sense. 



* " Anomodonts are not the parents of Mammals, but a collateral and closely 

 related group. The common parent of both may be sought in rocks older than 

 Permian, perhaps in Silurian or Devonian strata." Printed abstract of Professor 

 Seeley's argument, p. 3. 



