In the present state of knowledge, classification can be made only in a preliminary and tentative sort of way and no doubt differs widely from that which will eventually be reached. So far as the mammals are concerned, part of the problem would seem to be quite easy and part altogether uncertain. Some mammalian groups appear to be well defined and entirely natural assemblages of related forms, while others are plainly heterogeneous and artificial, yet there is no better way of dealing with them until their history has been ascertained. The mutual relations of the grand groups, or orders, are still very largely obscure. The class Mammalia is first of all divided into two subclasses of very unequal size. Of these, the first, PROTOTHERIA, is represented in the modern world by few forms, the so-called Duck-billed Mole (_Ornithorhynchus paradoxus_) and Spiny Anteaters (_Echidna_) of Australia. They are the lowest and most primitive of the mammals and retain several structural characters of the lower vertebrates. Their most striking characteristic is that the young are not brought forth alive, but are hatched from eggs, as in the reptiles, birds and lower vertebrates generally. The second subclass, EUTHERIA, which includes all other mammals, is again divided into two very unequal groups or infraclasses. One of these, DIDELPHIA, contains but a single order, the Marsupialia, or pouched mammals, now in existence, and is also very primitive in many respects, though far more advanced than the Prototheria. The young, though born alive, are brought forth in a very immature state and, with the exception of one genus (_Perameles_) the fœtus is not attached by a special structure, the placenta, to the womb of the mother. Like the Prototheria, the Marsupials, which were once spread all over the world, are at present almost entirely confined to Australia and the adjoining islands, the Opossums of North and South America, and one small genus (_Cænolestes_) in the latter continent being the exceptions to this rule of distribution. The second and vastly larger infraclass, the MONODELPHIA, is characterized by the _placenta_, a special growth, partly of fœtal and partly of maternal origin, by means of which the unborn young are attached to the mother and nourished during the fœtal period; they are born in a relatively mature state and are generally able to walk immediately after birth and resemble their parents in nearly all respects.