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only a few generalized marsupials, such as the American opossum,
have survived.
The Australian marsupials illustrate another point. The Australian
area is much smaller and less varied than the great land masses of
the northern hemisphere where the higher placentals evolved. There
is less scope for variation, less need for extremes of efficiency, so that
general selection-pressure never became so intense. As a result, the
Australian marsupials were not pushed so hard or so far along their
lines of specialization as were the placentals; they were not forced
to such a pitch either of biomechanical efficiency or of intelligence;
and they at once go downhill and are threatened with extinction
when they have to compete with introduced placcntal types.
Even more interesting are the recent studies on qualitative differ-
ences in the results of different kinds of selection, or, if you prefer, of
selection operating in different circumstances. Thus a peculiarly
acute competition takes place before birth among such mammals as
produce several young at a time. More eggs are always fertilized
than can survive to birth; there is thus an intra-uterine selection
which puts a premium on quick and vigorous growth, for any laggard
embryos will fail to get their fair share of the available nutriment and
will die and be resorbed or aborted. As J. B. S. Haldane has pointed
out, this pre-natal rapidity of growth will certainly tend to continue
after birth; and so the slow growth and prolonged infancy which
makes human learning possible could never have been evolved except
in a mammalian stock like that of the monkeys, where only one young
is normally born at a time.
Haldane has also drawn attention to the interesting point that in-
stinctive altruism, such as is shown by bees or ants, cannot possibly
be evolved except in social organisms where reproduction is confined
to a limited caste and the altruistic types are sterile.
The most far-reaching conclusion deriving from modern analysis,
however, is that the results of natural selection are not necessarily
beneficial to the species, and may even be harmful. This apparent
paradox is based on the fact that much of the struggle for existence
is not directed against the forces of nature, nor against enemies, nor
against competitors of other species, but against other members of the
same species. Not only does the species as a whole have to struggle
(in a metaphorical sense) to survive and reproduce, but so do the
individuals within it. In a given species of butterfly, for example,
only a small proportion of the young caterpillars will survive into the
butterfly stage. But among these, the decision as to which shall repro-
duce may depend on whether one can escape detection by its enemies