if the turbulent nature of the flow is examined in detail, we find that we need to use
probability distributions to describe the velocity, and even that is incomplete. We use
the simplified abstraction so often, and with such success, that we often forget the under-
lying complexity.
In biology, the uncertaintie s behind the abstractions are often closer to the surface. For
example, consider the concept of species. We would like to define species such that all
living things belong unambiguously to a species. This turns out to be impossible. One
definition is that a species is a group of organisms that interbree d with each other in nature
and produce healthy and fertile offspring. Thus, the horse and the donkey are different
species even though they can mate, because all their offspring are infertile. A number
of problems occur with this definition. First, not all organisms reproduce by breeding
(sexual reproduction). More important for this discussion is that there are populations
we would like to define into separate species that can interbreed, such as the domestic
dog and the African golden jackal. So perhaps we would alter the definition to include
organisms that can potentially interbreed. However, there are cases in n ature where organ-
ism A can breed with B, B with C, C with D, but D cannot bree d with A.
Similar problems occur every time we make a classification. The euglena is a single-
celled organism that can move at will through its aqueous environment. This motility,
together with its lack of a cell wall, would indicate that it should be classified as an ani-
mal. However, it has the green pigment chlorophyll, which enables it, like a plant, to cap-
ture light energy. Biologists have created a separate category, the protists, in part to
eliminate the problem of where to put the euglena. However, some protists, the algae,
are very similar to plants; others, such as protozoans, are animal-like. Thus, the classifica-
tions lack an iron-clad quality. Textbook definitions sometimes include the word mostly,
as in ‘‘animals are mostly multice llular.’’
Whether an organism is single-celled or multicellular is an important characteristic
used in classification. However, some may either be at different stages in their life
cycle or may simply change in response to environmental conditions. The slime mold
is an unusual organism that grows on the forest floor and behaves at one stage as a
mass of single-celled protozoans; at another stage the cells fuse into a single supercell
with many nuclei; and at yet another stage it forms fruiting bodies on stalks resembling
a plant.
Another idea that must be recognized as somewhat arbitr ary is the notion of an event.
This is another kind of useful fiction. In classical science (i.e., othe r than in quantum
mechanics), there are no events, only processes. Consider the ‘‘moment of conception,’’
when a sperm enters and fertilizes an egg. Exam ined more closely, we must realize that
the event consists of a sequence of changes. If we say that fertilization occurs as soon as a
sperm penetrates an egg’s cell membrane, we must ask, ‘‘penetrates how far?’’ If, instead,
we place the event at the moment that the chromosomes join into a single nucleus, we
must ask how complete the joining must be. It is like asking when two asymptotic curves
combine. The problem is not that science cannot say when the critical moment occurs. No
such moment actually exists.
Another thing that is important to appreciate about biology is that a certain amount of
caution is necessary when making predictions or judgments about the validity of reported
observations. Living things often surprise and contradict. The following quote by the
Dutch biologist C. J. Breje
`
r (1958) applies as well to biology as a whole: ‘‘The insect
world is nature’s most astonishing phenomenon. Nothing is impossible to it; the most
improbable things commonly occur there. One who penetrates deeply into its mysteries
AMBIGUITY AND COMPLEXITY IN BIO LOGY 7