580 J. T. WILSON AND J. P. HILL. 
ment of the immature young as in Monotremes, became further 
and more perfectly developed. We may further suppose 
that now one branch of the stock thus circumstanced, i.e. the 
Eutherian, went on to further exhaust the nutritive possibilities 
implied in the acquirement of a placental connection; retain- 
ing, however, and at the same time elaborating, the function 
of lactation in relation to more advanced stages of development. 
Another branch,—the Metatherian,—neglecting, from some 
cause or other, the opportunities offered by the original for- 
mation of a placental connection, may be supposed to have gone 
on developing the latent capabilities of the mammary function 
for the provision of adequate means of nutrition for the imma- 
ture young. These capabilities included the replacement of 
the primitive mammary pouches by a true Marsupium, partly 
at least derived from the latter. The original pouched condi- 
tion is thus to be carefully distinguished from the derivative or 
truly marsupliated condition of the Marsupialia, which is 
as much a secondary development along one line as, e. g., the 
ruminant mammary condition is along another. 
And the character of the marsupial dentition as we interpret 
it renders it very highly probable that the secondary evolution 
of a true Marsupium has all along been accompanied by a 
retrogressive development or degeneration from the normal 
early mammalian diphyodont dentition. Indeed, as Leche has 
pointed out, the functional adaptation of the mouth of the 
marsupial young to the peculiar suckling conditions prevalent 
in the order has no doubt conditioned the almost entire sup- 
pression of one of the two dentitions. The dentition sup- 
pressed is, according to Leche, the second (his third) ; but in 
our view it is, beyond all doubt, the first or milk dentition 
which has degenerated. 
University oF Sypvey, N.S.W.; 
July 4th, 1896. 
