Contributions to the theory of natural selection. A series of essays ... By Alfred Russel Wallace ...

ILLUSTRATIVE OF 1NATURAL SELECTIOPM. 139
its name, the possession of mammary glands and the
power of suckling the young. What more reasonable,
apparently, than to argue that the group in which
this important function is most developed, that in
which the young are most dependent upon it, and
for the longest period, must be the highest in the
Mammalian scale of organization? Yet this group is
the Marsupial, in which the young commence suckling
in a fcetal condition, and continue to do so till they
are fully developed, and are therefore for a long time
absolutely dependent on this mode of nourishment.
These examples, I think, demonstrate that we cannot settle the rank of a group by a consideration of
the degree in which certain characters resemble or
differ from those in what is admitted to be a lower
group; and they also show that the highest group of
a class may be more closely connected to one of the
lowest, than some other groups which have developed
laterally and diverged farther from the parent type,
but which yet, owing to want of balance or too great
specialization in their structure, have never reached
a high grade of organization. The Quadrumana afford
a very valuable illustration, because, owing to their
undoubted affinity with man, we feel certain that they
are really higher than any other order of Miammalia,
while at the same time they are more distinctly allied
to the lowest groups than many others. The case of
the Papilionidee seems to me so exactly parallel to
this, that, while I admit all the proofs of affinity
with the undoubtedly lower groups of Hesperidae and