mammals should have carried the eggs as long as possible in the
uterus. The embryo under these conditions would be better nourished
by a secretion of the uterine glands than by a very large amount of
yolk. The yolk would diminish and the egg decrease in size, and thus
the marsupial mode of development would have resulted. And, given
the marsupial mode of development and an embryo possessing an
allantois, it is almost a physiological necessity that in some forms
at least a placenta should develop. That the placenta has resulted
from some such process of evolution is proven by its different
stages of development in different orders of mammals. And even the
feeblest attachment of the allantois of the embryo to the wall of
the uterus would be of the greatest advantage to the species.

This is not the whole explanation; other factors still undiscovered
were undoubtedly concerned. But even this shows us that the internal
development of the young and the habit of suckling them was a
logical result of mammalian structure and position. The grand
results of this change we shall trace farther on.

The changes from the lower true mammals to the apes are of great
interest, but we can notice only one or two of the more important.
The prosimii, or "half apes," including the lemurs, are nearly all
arboreal forms. Perhaps they were driven to this life by their more
powerful competitors. The arboreal life developed the fingers and
toes, and most of these end, not with a claw, but with a nail. The
little group has much diversity of structure, and at present finds
its home mainly in Madagascar; though in earlier times apparently
occurring all over the globe. The brain is more highly developed
than in the average mammal, but far inferior to that of the apes.
They have a fairly opposable thumb.