DEVELOPMENT AND SUCCESSION OF TEETH IN PERAMELES. 431 
The views of Flower were subsequently adopted and ex- 
tended by Oldfield Thomas (5), who adduced in their support 
certain important paleontological facts. 
Upon the relation of the Eutherian to the Metatherian 
stocks, Thomas is notably explicit. He considers that “the 
change from Metatherian to Eutherian” occurred, “in all 
probability,’ within the limits of the family Dasyuride. 
His conclusions, therefore, that “the rudimentary tooth- 
change of Marsupials represents an early stage in the first 
formation ” of a complete change, and that the Marsupials 
are “still in a backward condition out of which the Kutheria 
have long since passed,” are of quite fundamental significance 
in his interpretation of the phenomena of the dentition of 
Mammals generally. 
The general argument in favour of Flower’s view of the 
primitive character of the single tooth-change in Marsupials, 
derived from the zoological position of the order, is expressed 
thus by Thomas :—“ When we consider that in every character 
of their organisation the Marsupials are infinitely behind and 
at a lower stage of evolution than the placental Mammals, it 
would appear to be a total subversion of all the ordinary rules 
to suppose that in this one character of their dentition they 
should have passed on in advance of all the other Mammals, 
and, having gone through the condition in which the latter 
now are, should have again nearly evolved away that process 
of tooth-change which is to its placental possessors so evidently 
advantageous ”’ (5, p. 450).! 
A more special argument is also derived from the fact “ that 
five out of the six families of Marsupials, natives both of 
Australia and America, have, with the comparatively unim- 
portant exceptions already noted as occurring among the 
Dasyuride, arrived at precisely the same stage of tooth- 
change ”—a circumstance which would be unlikely to occur if 
1 It may be remarked that this is precisely what has actually occurred in 
Dasyurus and Sarcophilus in relation to other Dasyuride, according 
to Thomas himself, i.e. after first evolving a tooth-change (single, of course), 
they have next proceeded to get rid of it. 
