In this chapter we consider what it means to be a marsupial. We do this by contrasting the scope of the marsupial and eutherian radiations, and examine hypotheses which attempt to explain why the marsupial radiation appears to be conservative. It is not our intention to enter into the sterile debate over the general advantages of ‘marsupialness’ and ‘eutherianness’. The coexistence of these groups in South America and Australia clearly testifies to the evolutionary viability of both.

However, we feel it is useful to point to a number of deficiencies in previous attempts to contrast the two taxa. Historically, it has often been assumed that marsupials are in some sense inferior to eutherians (Asdell, 1964; Lillegraven, 1975,1979), perhaps as a consequence of the widespread but fallacious belief that marsupials represent an intermediate grade of mammalian organisation between monotremes and eutherians. Both the original work and attempts to destroy the assumption (e.g. Kirsch, 1977a, b; Parker, 1977; Low, 1978), suffer from a lack of quantification and statistical examination, and have given rise to summary statements which are probably incorrect, for example, ‘the nonseasonal opportunism in most marsupials’ (Low, 1978, p. 206), or ‘even modest territorial behaviour in the marsupials is a comparative rarity’ (Lillegraven, 1979, p. 270).

There is a further underlying and unrecognised assumption which pervades this literature. This may be paraphrased: all aspects of an organism are perfectly tuned or adapted to their environments. As we shall see, the case for interpretation of species characteristics in an adaptive framework is by no means always clear, and the application of similar assumptions to the characteristics which distinguish higher taxa is even less justified.