452 
MR. 0. THOMAS OH THE HOMOLOGIES AND 
having passed through a complete cycle of evolution, the “ permanent ” tooth first 
developing, then having a milk-tooth superadded to it, and finally aborting itself and 
leaving its milk representative still persistent. 
These two questions answered, we come to the consideration of the phenomena 
observable during the growth of a milk-tooth above its permanent successor ; not the 
early and embryological ones, into which I do not propose to enter, but such as may 
be studied on the rich material of skulls of different ag’es available to me in the 
collection of the Natural History Museum. 
Taking now the skull of a tooth-changing Marsupial in which the first teeth are just 
appearing, we see that the single milk-premolar* comes up at about the same time as 
the true permanent premolars anterior to it; that the first molar quickly succeeds and 
the other molars follow, but that almost invariably, whether there is a change or not, 
the true pm 4 is considerably later in its development than the other teeth, and, 
especially, than either pm 3 or m 1 . In the original production, therefore, of a milk- 
tooth above one of the other teeth, say pm 3 , whose summit is, to commence with, 
fully equal in height at all stages of development to the summit of the milk-tooth of 
pm 4 standing just behind it, we see that a change of position is necessary in this pm 3 
before a milk-tooth can be developed over it, a change which can apparently only be 
brought about by the retardation of its growth, and its approximation thereby to the 
state in which pm 4 now is. 
Should this supposition be true, we should expect to find that, anterior to the first 
appearance of a milk-tooth in any group, specimens would be found showing a 
preliminary retardation of the individual teeth over which, in a later generation, 
milk-teeth were to be developed. The difficulties in the way of understanding how 
the ordinary processes of evolution can have first brought about such a preliminary 
retardation are, of course, considerable, unless it be that the retardation is itself a 
favourable character, by its preventing the undue crowding of the young animals 
mouth, while, at the same time, the full number of teeth are developed for use by 
the adult. In this case it would be comparatively easy to understand the assumption 
of a milk dentition by the two steps, each favourable in itself—(1) of a retardation 
for packing purposes of the permanent tooth in some large-toothed form, followed, in 
a later generation, by (2) the temporary development of a deciduous tooth in one of 
its descendants with the teeth so small that the gap in the tooth-row during youth, 
inherited from large-toothed ancestors, had become a defect to be remedied in this 
most effective manner. 
Turning now to the actual facts, we find that there is among the Marsupials a 
* It seems better to use this term ratber than “ milk-molar,” as the milk-teeth have nothing to do 
with the true molars, and that name is, therefore, productive of constant confusion. And, again, the 
use of the word “ deciduous ” instead of “ milk ” seems to be inadvisable, as both the molars and pre¬ 
molars of the second set are often themselves deciduous, while occasionally those of the first or milk set 
remain throughout life. 
