432 J. I. WILSON AND J. P. tbh, 
the modern tooth-change were a remnant of a fuller one, for 
then “ we should naturally expect that, under the very various 
conditions of the struggle for existence, equally various 
degrees of reduction would have been attained to.” It may 
be pointed out that the cogency of this argument would 
entirely disappear if it could be shown to be probable that the 
(hypothetical) reduction was dependent upon conditions of 
life common to, and peculiar to, the entire group. 
In professing his firm adherence to Flower’s view that the 
teeth of Marsupials in front of the last premolar represent the 
permanent teeth of other Mammals, Thomas states that he 
was led to that opinion by “ finding the impossibility of work- 
ing out the general homologies of the teeth on the basis of 
the opposite view,” and by very extended observations of 
specimens. 
It is perhaps unnecessary to follow Mr. Thomas through 
his somewhat elaborate and rather far-fetched doctrine of a 
retardation which ought to have occurred in Marsupials 
if their teeth had formerly possessed milk predecessors which 
were subsequently lost,—a retardation parallel to that which 
occurs in the case of the last permanent premolar; and which 
he alleges as also occurring in the case of the first incisor in a 
number of Marsupials, probably by way of preparation for the 
acquirement of a milk-tooth. Thomas himself remarks upon 
‘the difficulties in the way of understanding how the ordinary 
processes of evolution” could first have “ brought about such 
a preliminary retardation,’—a remark with which his readers 
will readily agree. 
The above views have now an interest for us which is largely 
historical, since Thomas has explicitly surrendered the main 
point of his position in view of later discoveries. 
The views of Flower, prior at least to Thomas’s advocacy of 
them, had not as a whole received general approval. His 
identification of the non-changing antemolar teeth of Marsu- 
pials with those of the permanent series of Placentals was, 
indeed, practically universally received. The view, however, 
that the seemingly almost complete monophyodontism of 
