As regards the structure of their skull, the constitution of the pectoral arch, and their persistence in the phase (embryonic in other mammals) in which the rectum and the urinary and genital ducts open into a single cloaca, the Monotremata (Ornithorhynchus, Echidna), limited to Australia and Tasmania, are the lowest members of their class, and must be considered as remnants of a division reaching from indeterminable past ages down to the present time. It may be presumed that the Marsupials were developed from an analogous grade. Their powers of adaptation have been chiefly testified in Australia, where the subdivisions of the order, usually designated as families, are, in dentition and habits of life, developed in a manner analogous to several of those orders which appear on the second great scene of mammalian development, namely, the Northern hemisphere.

Far advanced beyond the Monotremata as to skeleton, they remain on a low grade with respect to the reproductive system, and are implacental, like the Monotremata. That is to say, the embryonic blood-vessels do not enter into those close relations with the blood-vessels of the maternal ovary, by which the more perfect development of other mammals within the mother's womb is effected. This character and the correlative formation of the pouch in which to carry the immaturely born offspring, bind together the various families of Marsupials, which deviate from one another like other orders.

With the exception, therefore, of the two orders named, in all mammals the embryo is attached to the maternal organism by the so-called placenta. The blood-vessels of the developing offspring which reach the wall of the uterus by the intervention of the allantois, form coils

and loops, between which grow similar offshoots and appendages of the blood-vessels of the ovary, so that through the walls of the contiguous blood-vessels an abundant exchange of fluids takes place between the two, and therewith a prolonged nutrition and a further and more complete development of the fœtus. The higher character of the placental mammals, usually plainly evinced by their anatomical relations, is thus based on the existence of the placental mass. All intermediate grades are, however, wanting which would entitle us to infer with certainty the direct transition from implacental to placental mammals. The Edentata, (Bruta), manifestly the lowest of placental mammals, are so devoid of any nearer morphological relations with the Marsupials, that we must needs be content to assume generally, on these indications, supported by geographical distribution and geology, that they represent a very ancient branch of the placental mammals. As we saw in the tenth chapter, they are scattered remnants which can only by compulsion be united into a single order. Sloths, armadilloes, ant-eaters, differ from one another at least as much as rodents, insectivora, and bats. The doctrine of Descent is not discredited because it is unable to account for these fragments of bygone animal life, but in the absence of data it is for the time in presence of an impossibility.

To ascertain the relationships of other orders, the modern systematizers, and also the supporters of the system of Descent, have thought fit to lay great stress on the presence or absence of the so-called decidua. This requires a short explanation. In many orders of

mammals, the vascular processes and vili of the wall of the ovary become so closely connected with the foetal portion of the placenta, that at birth the entire membranous coating of the ovary is detached and thrown out with it. In others, the vascular villi are not so closely adherent; they yield without important lacerations, and hence no deciduous membrane (Membrana decidua) is ejected. Now, as it appears to me, the specific conditions of the formation of the decidua have been far too little compared to justify our inferring any close affinity from the mere fact that portions of the coating of the ovary are lost in parturition. Much rather it must be unreservedly admitted that the formation of decidua might be occasioned by subordinate circumstances of the most varied kinds, and hence in orders only remotely allied, or allied merely as placental mammals. We therefore consider the decidua to be a subordinate systematic feature where anatomical and morphological reasons are opposed to it.

We go yet further. In the modern system the form of the placenta is likewise employed in the grouping of organisms. If among the Deciduata, lemurs, rodents, insectivora, bats and monkeys, are classed together as orders with discoidal placenta, this combination is certainly supported by a series of other reasons, and it is quite probable that within this group of orders the form of the placenta is due to homology, that is to Descent. But when beasts of prey, elephants and the Daman (Hyrax) are further cited as orders with zonary placenta, we find ourselves in the same position as when the decidua was reckoned decisive as to the closer affinity; and we are of opinion that the subordinate form

of the placenta might similarly arise in different ways, just as it has been variously developed in the well-substantiated division of the Ungulata. To corroborate our view by example, we are certainly unable to make any positive statements as to the derivation of the Proboscida. It is, however, none the less certain that nothing positive is implied by the customary classification by reason of their zonary placenta. But we shall more nearly approach the truth if we place this branch of unknown origin typically nearer to the Ungulata than to the beasts of prey. If, moreover, as non-deciduate mammals, the Cetacea are held to be more closely allied to the Ungulata than to the Carnivora, which are deciduate,-in our eyes, this circumstance is not decisive, as more important reasons argue that the Cetacea were first developed from carnivorous genera.

In our exposition of the geographical distribution of animals, we derived instruction from Rütimeyer with

reference to the relationships of the Ungulata - in particular. In no other division do we possess such