To review the objections which have been raised against these views would be entering on a very difficult question, and one in regard to which uniformity of opinion by no means exists among naturalists even at the present day. It may be mentioned, however, that from the circumstance of the later milk-premolars resembling (as was noticed by Flower in the case of the one tooth replaced in marsupials) the true molars rather than the permanent premolars, it has been suggested that the milk-dentition is serially homologous with the true molars. And on this view, the entire dentition of marsupials (with the exception of the one replacing tooth) corresponds to the milk-dentition of placentals. Possibly, however, the larger number of incisors which distinguish many of the carnivorous marsupials from the placentals may be due to the development of teeth belonging to the permanent series with those of the milk-set, and both persisting together throughout life. Be this as it may, it is evident, on the above view of the serial homology of their dentition, that marsupials, instead of as Flower supposed, showing the commencement of a milk-dentition, really exhibit the decadence of the permanent series. In this respect they display a precise similarity to the modern elephants, as indeed was pointed out by Flower in his original paper, although on a false premiss, for he at that time regarded the anterior cheek-teeth of the elephant as the representatives of the permanent premolars, whereas they really correspond with the milk-premolars. One objection has indeed been raised with regard to the identification of the adult marsupial dentition with the milk-set of placentals, namely, the existence in certain marsupialia of rudimentary teeth belonging to an earlier set than the one functionally developed. This has been got over by regarding these rudimentary germs as the representatives of a prelacteal series.