SUCCESSION OE THE TEETH IN THE MARSUPIALI A. 
639 
In the preceding account I have used the term “ permanent” for those teeth which 
remain in use throughout the animal’s life, or, if they fall out (as with the rudimentary 
canines and the premolars of the Macropodidce), do not give place to successional teeth, 
and I have therefore assumed that the milk or temporary dentition of the typical diphy- 
odont mammals is represented in the Marsupials only by the deciduous molars. 
It may be held, on the other hand, that the large majority of the teeth of the Marsu- 
pials are the homologues of the milk- or first teeth of the diphyodonts, and that it is the 
permanent or second dentition which is so feebly represented by the four successional 
teeth. This view is supported by many general analogies in animal organization and 
development, such as the fact that the permanent state of organs of lower animals often 
represents the foetal or transitional condition of the same parts in beings of higher 
organization. 
Looking only to the period of development of the different teeth in some of the mar- 
supial genera, we might certainly be disposed to place the successional premolar in a 
series by itself, although, indeed, all its morphological characters point out its congruity 
with the row of teeth among which it ultimately takes its place, the reverse being 
the case with its predecessor. 
It is, however, almost impossible, after examining the teeth of the young Thylacine 
(Plate XXX. fig. 6), to resist the conclusion originally suggested. The unbroken series 
of incisors, canines, premolars, and anterior true molars of nearly the same phase of deve- 
lopment, with posterior molars gradually added as age advances, form a striking contrast 
to the temporary molars, so rudimental in size and transient in duration. I can scarcely 
doubt but that the true molars of this animal would be identified by every one as homo- 
logous with the true molars of the diphyodonts, which are generally regarded as belonging 
to the permanent series, although they never have deciduous predecessors. Now, if the 
homology between the true molars of the Thylacine and those of a Dog, for instance, be 
granted, and if the anterior teeth (incisors, canines, and premolars) of the Thylacine be 
of the same series as its own true molars, they must also be homologous with the corre- 
sponding permanent teeth of the Dog. 
It may be objected to this argument, that the true molars of the diphyodonts, not being 
successional teeth, ought to be regarded as members of the first or milk-series ; but, in 
truth, the fact that they have themselves no predecessors does not make them serially 
homologous with the predecessors of the other teeth, while their morphological charac- 
ters, as well as their habitual persistence throughout life, range them with the second 
or permanent series. 
We have been so long accustomed to look upon the second set of teeth as an after 
development or derivative from the first, that it appears almost paradoxical to suggest 
that the milk or deciduous teeth may rather be a set superadded to supply the tempo- 
rary needs of mammals of more complex dental organization. But it should be remem- 
bered that, instead of there being any such relation between the permanent and milk- 
teeth as that expressed by the terms “ progeny ” and “ parent” sometimes applied to 
mdccclxvii. 4 s 
