450 
MR. 0. THOMAS OH THE HOMOLOGIES AHD 
modified into the very various forms of modern dentition, and especially as to the 
passage from the Marsupial to the Placental style of dentition. 
The first and most fundamental question that arises is this :—Is the rudimentary 
tooth-change now found in the Marsupials the last remnant of a complete change present 
both in their ancestors and in those of the Placentalia, or does it represent an early 
stage in the first formation of such a complete change, the Marsupials being still in a 
backward condition, out of which the Eutheria have long ago passed % 
To my mind it is perfectly clear that it is the second and not the first question 
that should be answered in the affirmative ; although, so far as I can find, all the 
Continental and many of the English naturalists think the opposite—a view, however, 
that, although easy and obvious at first sight, I cannot for one moment believe to be 
correct. 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. It would be 
to my mind inconceivable that this should be the case, considering how universal 
among the Eutheria a more or less complete tooth-change is, and how useful it has 
proved to be to them, as evidenced by the very fact of their so wholly supplanting 
the more lowly organised Marsupials—more lowly organised in their dental as well as 
in their other characters, and not further advanced, as would have to be presumed 
were their teeth looked upon as a later development of a fully Diphyodont set. 
And again, the mere fact that five out of the six families of Marsupials, natives 
both of Australia and America, have, with the comparatively unimportant exceptions 
already noted as occurring among the Dasyurkke, arrived at precisely the same stage 
of tooth-change is itself a very strong argument in favour of the view now advocated ; 
for, were the modern tooth-change a remnant of a fuller one, 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. On the other hand, we 
should be most unlikely to find, as is now the case, more than 90 per cent, of the 
existing and fossil Marsupials changing one single tooth, and one only, and the small 
remainder merely differing from them in a direction away from and not towards that 
fuller tooth-change of which it is said their ancestors were once possessed. 
Of the one form of evidence needed to give any weight to the opposite theory, 
namely, that derived from Palaeontology, there seems not to be one atom, no fossil 
Marsupials having ever been found showing traces of a larger amount of tooth-change 
than the recent ones, nor do I think such a discovery likely to be made, as, to the 
best of my belief, no such animal has ever existed. This view is very much 
strengthened by the fact that, as already noted, one of the oldest Mammals known, 
