1867.] and Succession of the Teeth in the Marsupialia. 4<67 



first or milk- series ; but, in truth, the fact that thev have themselves no 

 predecessors does not make them serially homologous with the prede- 

 cessors of the other teeth, while their morphological characters, 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 temporary needs of mammals of more complex 

 dental organization. But it should be remembered that, instead of there 

 being any such relation between the permanent and the milk-teeth as that 

 expressed by the terms "progeny'* and "parent" (sometimes applied to 

 them), they are both (if all recent researches into their earlier development 

 can be trusted) formed side by side from independent portions of the pri- 

 mitive dental groove, and may rather be compared to twin brothers, one of 

 which, destined for early functional activity, proceeds rapidly in its develop- 

 ment, while the other makes little progress until the time approaches when 

 it is called upon to take the place of its more precocious locum tenens. 



Many facts appear to point to the milk-teeth as being the less constant 

 and important of the two sets developed in diphyodont dentition. Among 

 these the most striking is the frequent occurrence of this set in a rudimen- 

 tary and functionless or, as it were, partially developed state. The milk- 

 premolars of some Eodents (as the Guinea-pig), shed while the animal is 

 in utero, the simple structure and evauescent nature of the milk-teeth of 

 the Bats, Insectivores, and Seals, the diminutive first incisors of the 

 Dugongs and Elephants, all appear to be cases in point. On the other 

 hand, examples of the commencing or sketching out, as it were, of the 

 successors to a well-formed, regular, and functional first set of teeth, are 

 rarely, if ever, met with. Occasional instances of the habitual early deca- 

 dence, or, perhaps, absence of some of the second or so-called permanent 

 teeth occur in certain animals ; but these are rather examples of the disap- 

 pearance or suppression of organs of which there is no need in the economy, 

 and chiefly occur in isolated and highly modified members of groups in 

 the other members of which the same phenomenon does not occur, as in 

 Cheiromys among the Lemurs, Trichechus among the Seals, and the 

 recent Elephants (as regards the premolars) among the Proboscideans. 

 They form no parallel to the cases mentioned above of the rudimentary 

 formation of an entire series of teeth of the temporary or milk-set. 



To return to the marsupials : — If this view be correct, I should be quite 

 prepared to find, in phases of development earlier than those yet examined, 

 some traces either of the papillary, follicular, or saccular stages of milk- 

 predecessors to other of the teeth besides those determinate four in which, 

 for some reason at present unexplained, they arrive at a more mature 

 growth*. Such proof as this would alone decide the truth of these specu- 

 ^ It may be remarked that the milk-tooth, which alone is developed in the Marsu- 



