582 CHAPTER 22world; so, as the philosopher Leibniz said, this must be “the best of all possible
worlds.” (His phrase was mercilessly ridiculed by Voltaire in his satire Candide.)
The adaptive design of organisms, in fact, has long been cited as evidence of an
intelligent designer. This was the thrust of William Paley’s famous example: as the
design evident in a watch implies a watchmaker, so the design evident in organ-
isms implies a designer of life [88]. This argument from design has been renewed
in the “intelligent design” version of creationism, and it is apparently the most
frequently cited reason people give for believing in God [90].
Of course, Darwin made this particular theological argument passé by provid-
ing a natural mechanism of design: natural selection. Moreover, Darwin and sub-
sequent evolutionary biologists have described innumerable examples of biologi-
cal phenomena that are hard to reconcile with beneficent intelligent design. Just as
Voltaire showed (in Candide) that cruelties and disasters make a mockery of the idea
that this is “the best of all possible worlds,” biology has shown that organisms have
imperfections and anomalies that can be explained only by the contingencies of
history, and characteristics that make sense only if natural selection has produced
them. If “good design” were evidence of a kindly, omnipotent designer, would “infe-
rior design” be evidence of an unkind, incompetent, or handicapped designer?
Only evolutionary history can explain vestigial organs—the rudiments of once-
functional features, such as the human appendix, the reduced wings under the
fused wing covers of some flightless beetles, and the nonfunctional stamens or
pistils of plants that have evolved separate-sexed flowers from an ancestral her-
maphroditic condition. Only history can explain why the genome is full of “fossil”
genes: pseudogenes that have lost their function.
Because characteristics evolve from pre-existing features, often undergoing
changes in function, many features are poorly engineered, as anyone who has suf-
fered from lower back pain or wisdom teeth can testify. Once the pentadactyl limb
became developmentally canalized, tetrapods could not evolve more than five dig-
its even if they would be useful: the extra “thumb” of the giant panda and of moles
is not a true digit at all, and it lacks the flexibility of true fingers because it is not
jointed (see Fig ure 20.9). And it is a pity that humans, unlike salamanders, cannot
regenerate lost limbs or digits.
If a designer were to equip species with a way to survive environmental change,
it might make sense to devise a Lamarckian mechanism, whereby genetic changes
would occur in response to need. Instead, adaptation is based on a combination
of a random process (mutation) that cannot be trusted to produce the needed
variation (and often does not) and a process that is the very epitome of waste and
seeming cruelty (natural selection, which requires that great numbers of organ-
isms fail to survive or reproduce). It would be hard to imagine a crueler instance
of natural selection than sickle-cell disease, whereby part of the human popula-
tion is protected against malaria at the expense of countless other people, who
are condemned to die because they are homozygous for a gene that happens to be
worse for the malarial parasite than for heterozygous carriers (see Chapter 5). A nd,
of course, this process often does not preserve species in the face of change: more
than 99 percent of all species that have ever lived are extinct. Were those species
the products of an incompetent designer? Or one that couldn’t foresee that species
would have to adapt to changing circumstances?
Many species become extinct because of competition, predation, and parasit-
ism. Some of these interactions are so appalling that Darwin was moved to write,
“What a book a devil’s chaplain might write on the clumsy, wasteful, blunder-
ing, low, and horribly cruel works of Nature!” Darwin knew of maggots that work
their way into the brains of sheep, and of wasp larvae that, having consumed the
internal organs of a living caterpillar, burst out like the monster in the movie Alien.
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