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Updated: June 24, 2025
What is known as Wallace's line corresponds with the deep channel running between the islands of Bali and Lombok and continuing northwards to the west of Celebes. On the eastern side the fauna are non-Asiatic. Yet somehow into Australia with its queer monotremes and marsupials entered triumphant man man and the dog with him.
These limbless reptiles, evolved by degeneration from lizardlike ancestors, appeared in nonpoisonous types scarcely to be distinguished from those of the present day. The higher mammals had appeared, and henceforth the lower mammals the monotremes and the marsupials are reduced to a subordinate place. These first true mammals were archaic and generalized in structure.
And the Monotremes are certainly one of the surviving forms of the intermediate stages. The next higher step in the ascent of Mammal life above the Monotreme is occupied by the Marsupials, or milk-giving, pouched animals, of which family the opossum and kangaroo are well known members.
The cochlea is wanting even in the Monotremes, and is restricted to the rest of the mammals and man. The auditory nerve, or eighth cerebral nerve, expands with one branch in the cochlea, and with the other in the remaining parts of the labyrinth.
A similarly increasing ratio should be granted for the successive developments of the difference between the Lemuroid and the higher forms of primates; for those between the original primate and other root-forms of placental mammals; for those between primary placental and implacental mammals, and perhaps also for the divergence of the most ancient stock of these and of the monotremes, for in all these cases modifications of structure appear to increase in complexity in at least that ratio.
But among the mammals this cloaca is only permanent in the Monotremes, as it is in all the birds, reptiles, and amphibia. The use that we have hitherto made of our biogenetic law will give the reader an idea how far we may trust its guidance in phylogenetic investigation.
In this respect there is a very conspicuous difference between the lower and higher mammals. In the Marsupials, in which the embryo is only a comparatively short time in the womb and is born in a very immature condition, the vascular arrangements in the yelk-sac and the allantois suffice for its nutrition, as we find them in the Monotremes, birds, and reptiles.
Scientists are divided in theories as to whether the Monotremes are actually descended directly from the Reptiles or Birds, or whether there was a common ancestor from which Reptiles and Birds and Mammals branched off. But this is not important, for the relationship between Reptiles, Birds and Mammals is clearly proven.
First among the mammals we find the aplacentals, or those which bring forth immature young, which are grouped into two divisions, i.e., the monotremes, or one-vented animals, in which group belong the duck-bills, spiny ant-eaters, etc.; and the marsupials, or pouched animals, in which group belong the kangaroo, opossum, etc.
These primitive mammals help us to reconstruct the mammal life of the Mesozoic Epoch. The bones that we have are variously described in geological manuals as the remains of Monotremes, Marsupials, and Insectivores. On the other hand, the lower jaw of the Marsupial is characterised by a peculiar hooklike process, and this is commonly found in Mesozoic jaws.
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