SUCCESSION OF THE TEETH IN THE DASYUR1DH1. 
451 
the Mesozoic Triacanthodon, had precisely the same, and no more, tooth-change than 
the modern Marsupials. 
For these various reasons, therefore, we may, I think, take it for granted that the 
ancestors of the Marsupials never had at any time a more complete change of 
dentition than they have now, and that they arrived at their present state at an 
immensely early period, since which time they have as a whole practically stood 
still, except that a few isolated forms have, comparatively recently, lost again even 
the small amount of change they once possessed. 
The second question, and one equally vital, is as to whether the milk set of teeth 
is the original primary set with the permanent one superadded to it, as believed by 
many naturalists, and especially by embryologists, or vice versd: a question with which 
Professor Flower has dealt in the first of his papers above referred to. # His 
conclusion was that the permanent dentition was the original one, and that the milk 
set had been afterwards developed as an addition to it—a view to which, although 
inclined at first to disagree, I have now become a firm adherent. To this opinion I 
have come by finding the impossibility of working out the general homologies of the 
teeth on the basis of the opposite view, and by the comparison of an infinitely larger 
number of specimens of various sorts than even Professor Flower had access to. 
The chief cause of the prevalence of the opinion that the permanent dentition is a 
later development than the milk is the deceptive appearance presented in the early 
stages of tooth-development, when the germ of the permanent tooth is first seen as 
a bud growing out from that of the milk-tooth, whence it has naturally been supposed 
that the latter was the primary and the former the secondary development. Even 
should this appearance of budding off, however, be entirely correct—and the fact 
itself is strongly denied by R. BaumeI —it may be argued that, considering the 
uniform direction of the evidence drawn from later stages, there is no sufficient reason 
to deny the possibility of a secondary organ, whose very raison d’etre, as in the case 
of the milk-tooth, is its speedy and precocious development, so overshadowing in 
size and rapidity of growth what is really the primary as to make the latter appear 
as its bud, and therefore, although falsely, as a secondary and subsidiary growth. 
A second, apparently adverse, argument may be drawn from the few instances 
known of milk-teeth J developed, and remaining through life, without having, or 
only rarely having, true permanent successors, as in the anterior premolars of the 
Proboscidea § and some of the Perissodactyla, || but these are clearly due to the teeth 
* ‘ Phil. Trans.,’ 1867, p. 639. 
f * Odontologische Forschungen : Versuch einer Entwickelungsgeschichte des Gebisses,’ 1882, p. 75. 
t I.e., the homolognes of the corresponding milk-, and not permanent, teeth in the allied species. 
§ R. Lydekker, ‘ Cat. Foss. Mamm. Brit. Mus.,’ Part 4, 1886. ( Introduction , p. vii.) 
|| R. Lydekker, ‘ Bengal, Asiat. Soc. Journ.,’ vol. 49, 1880, p. 135. This interpretation by Mr. 
Lydekker of the homologies of the non-changing pm 1 in the Rhinoceroses has been called in question, 
but his evidence, drawn from an abnormal Rhinoceros skull in the Calcutta Museum, seems to me to be 
fully sufficient to support the conclusions he came to—conclusions with which all the specimens that I 
have seen quite agree. 
