It is that osteology must after all constitute the core of the true theory of mammalian history. It is only by means of the skeleton that we are able to correlate the knowledge of living with that of fossil mammals and thus to synthesize the results of palaeontology, systematic mammalogy and comparative anatomy.

It will, perhaps, be helpful for an understanding of marsupial conditions if it be explained that Huxley, Dollo, Bensley, and other authorities on this group have been convinced that the Metatheria were derived from an ancestor sufficiently specialized for an arboreal existence for the latter adaptation to have left a lasting impression upon the foot structure. This is in contrast to the protoplacental ancestor, which, although presumably to some extent arboreal in habit, was hardly modified in this direction to a very definite degree.