. 270 THE DOCTRINE OF DESCENT 
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 mem- 
bers 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 mam- 
malian development, namely, the Northern hemisphere, 
Far advanced beyond the Monotremata as to skeleton, 
they remain on a low grade with respect to the repro- 
ductive system, and are implacental, like the Monotre- 
mata. 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 develop- 
ment 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 off- 
spring, 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 
