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The Eighth Lesson: The Ascent of Man.831
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 characteristic
feature of this family of creatures is the possession of an external pouch in
the female, in which the young are kept and nourished until they can take
care of themselves as the young of other animals are able to do. The young
of the Marsupials are brought forth, or born, in an imperfect condition, and
undeveloped in size and strength. There are fossil remains of Marsupials
showing that in past ages creatures of this kind existed which were as large
as elephants.
In the more common form of Mammals the young are brought forth fully
formed, they having received “nourishment, before birth, from the mother’s
body, through the placenta, the appendage which connects the fetus with
the parent. The Placental Mammals were the best equipped of all the lifeforms
for survival and development, for the reason that the young were
nourished during their critical period, and the care that the mammal must of
necessity give to her young operated in the direction of affording a special
protection far superior to that of the other forms. This and other causes
acted to place the Placentals in the “Royal line” from which Man was evolved.
The following families of Placental Mammals are recognized by Science,
each having its own structural peculiarities:
The Edentata, or Toothless creatures, among which are the sloths, anteaters,
armadillos, etc. These animals seem to be closer to the Monotremes
than they are to the Marsupials;
The Sirenia, so called by reason of their fanciful resemblance to the sirens
of mythology, among which are the sea-cows, manatees, dugongs, etc.,
which are fish-like in structure and appearance, the fore-limbs being shaped
like paddles, or fins, and the hind-limbs being absent or rudimentary;
The Cetacea, or Whale Family, including whales, Porpoises, dolphins, etc.,
which are quite fish-like in appearance and structure, their forms being