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428 appendix
when young and blind, eject the other hatchlings out of
the nest, and the parent birds, of a different species, raise
the young cuckoo. Cowbirds also exhibit such behavior;
and among cowbird species, we see all the intermediate
stages ranging from occasionally laying the eggs in the
nests of other bird species, to depending upon this strategy
entirely. In one example, the cowbird seems to employ an
intermediate strategy that does not work very well.
2. Some ant species enslave other ant species by raiding
their nests and stealing pupae, then raising the ants of the
other species as slaves. We see here, also, the entire range
of adaptations. In some ant species, the masters do much
of the work, forcing the slaves to help them; for example,
when it comes time to move to a new nest, the masters
carry the slaves. In some of these, the slaves accompany
the masters on food-gathering expeditions; in others, they
stay at home. At the other end of the spectrum, there are
ant species in which the masters are entirely dependent
upon the work of the slaves and cannot even feed themselves
without the help of the slaves. At moving time, the
slaves carry the masters.
3. Honeybees make the wax chambers of their nests entirely
by instinct. These chambers appear complex: The hexagonal
form of the chambers is the mathematically perfect
compromise of strength and economy of space. “He must
be a dull man who can examine the exquisite structures of
a comb, so beautifully adapted to its end, without enthusiastic
admiration.” Although the comb-building instinct
appears complex, it results from a few basic instincts, and
the intermediate stages of its evolution can still be found
represented in other species of bees. “Let us look to the
great principle of gradation, to see whether Nature does
not reveal to us her method of work.”
• We find, among bee species, the whole range of honeycomb
complexity, from the simple round chambers of
bumblebees to the complex hexagonal chambers of honeybees,
with other species intermediate.
• The complexity of beehives results from the repeated
application of a few basic behaviors, which I demonstrated
by experiments in which I provided different
starting conditions for the bees to build their chambers
and observed what they did. It is quite simple, actually.
“The work of construction seems to be a sort of balance
struck between many bees, all instinctively standing
at the same relative distance from each other, all
trying to sweep equal spheres, and then building up, or
leaving ungnawed, the planes of intersection between
these spheres.” The bees are not intelligent architects;
they follow simple instincts and in so doing build up
complex structures. [This is an example of what is now
called emergence.]
The intermediate stages of honeycomb evolution would
have proved advantageous; for wax is a very expensive material
for the bees to make, and any adaptation that helped
them economize its use would be selected. “Thus, as I believe,
the most wonderful of all known instincts, that of the hivebee,
can be explained by natural selection having taken
advantage of numerous, successive, slight modifications of
simpler instincts.”
One objection to my theory at first appears fatal. In some
social insects—including honeybees—the worker individuals
are sterile. How could natural selection favor characteristics
in the worker bees that they themselves cannot transmit to
the next generation? Clearly the answer is that the characteristics
are transmitted by the queen bee and drones. Natural
selection works on families, not just individuals. A livestock
breeder who finds that a certain animal has desirable traits,
but discovers this only after slaughtering the animal, can still
breed for those traits by using the dead animal’s closest relatives
as the breeding stock. A division of labor within a society
of ants, even if this division includes some of the members
being sterile, is beneficial to the ants as surely as a specialization
of labor is of benefit within human society. [Today this
is recognized as inclusive fitness; see altruism.]
chapter 9. Hybridism
Scientists commonly believe that crosses between varieties
within a species (as of different breeds of pigeon) produce
fertile offspring, whereas the crosses between different species
(as when horse and donkey cross to make a mule) produce
sterile offspring (see hybridization). If this is an unbreakable
rule of nature, then natural selection could never transform
varieties into species! But this pattern, while commonly
observed, is not a rule, for these reasons:
1. Crosses between varieties, and between species, produce
all different gradations of fertility and sterility. It is not
true to say that crosses between varieties are perfectly fertile
while those between species are completely sterile. In
some plants, interspecific crosses produce offspring even
more vigorous and fertile than crosses within a species!
The details of which varieties or species can be interbred
with which others, and which cannot, seem unrelated to
how different or similar the varieties or species appear to
one another. In some cases, crossing two species yields different
results, if one rather than the other species is used
as the father (compare, for example, mules and hinnies).
Sometimes two species can be easily crossbred but they
produce few fertile offspring, while sometimes two species
can be crossbred only with difficulty, but the resulting offspring
are vigorous and fertile. There is no simple rule that
varieties can cross and species cannot. You simply cannot
tell which crosses will yield good offspring and which will
not, until you try the crosses yourself.
2. Some of the studies of crossbreeding are flawed because
they are performed with livestock or with crop plants that
have experienced many generations of inbreeding, which
itself results in reduced reproductive vigor.
3. It is not surprising that, according to some naturalists,
varieties can cross while species cannot, because they
define species that way! If two forms cannot cross, they
are defined as different species. This is circular reasoning.
[For a discussion of the complexities surrounding
what is now called the “biological species concept,” see
speciation.]