KINGDOMS 1
-Jack R. Holt
AN EPIPHANY
Then all at once I see it and I know at once what it is: epiphany.
-James Joyce (Stephen Hero)
One time while looking through a microscope at some amazing things, Lois Pfiester 2
watched a cell creep along the outside of an algal filament. Those of us in the lab
watched it as it perched itself on the filament and then “sucked” the contents into itself,
leaving an empty cell wall with a small round hole in it. After we had watched it for
some time, the professor looked up from her microscope and said, almost as an epiphany
that these things were not plants, but protists.
Figure 1. A dinoflagellate taken with a light microscope. The cell has a wall-like armor
around it and is photosynthetic (both plant-like characteristics). The squiggly line that
trails off to the lower right is a flagellum, a whip-like structure that allows the cell to
move (an animal-like characteristic).
Figure 2. A dinoflagellate taken with a Scanning Electron Microscope. It has attached
to a surface and is feeding on bacteria there (an animal-like characteristic).
1
This is a revision of an essay that I wrote in 1999 and published in a collection called Paths of Science in
2001.
2
Consult my essay Killer Algae for a short biography of Lois Pfiester and a description of dinoflagellates.
1
That is exactly the problem that I had been wrestling with since I had begun graduate
school in 1973. I had studied a group of organisms called dinoflagellates in the
Department of Zoology at the University of Oklahoma. There, they were called animals
in a group of lower animals called the protozoa because they swim. By 1976 I had
moved over to the Department of Botany and Microbiology at the same university where
dinoflagellates were algae, a group of lower plants because they were photosynthetic and
had a cell wall (see Figure 1). Later, I worked on a species that was photosynthetic and
swam in the plankton during the day, but at night, it attached to a substrate and fed on
bacteria (see Figure 2). Thus, dinoflagellates seemed to be neither plants nor animals but
protists, a term that denoted things that generally were single-celled and included a
variety of organisms. Some were claimed by Botanists, some by Zoologists and some by
Mycologists (those who study fungi). In the Protists, however, terms like higher and
lower made no sense. Indeed, where did such terms come from?
CLIMBING THE LADDER
There is observed a continuous scale of ascent toward the animal.
-Aristotle
Ask most people about the different kinds of living things and they will probably
answer Plants and Animals. In many large institutions such as the University of
Oklahoma where Lois Pfiester taught, Biology is divided into Botany (plants) and
Zoology (animals). This “reasonable” differentiation is a reflection of an ages old
dichotomy that was formalized by the philosopher Aristotle of Stagira (384-322 B.C.;
Figure 3). He described a Scala Naturae (ladder of nature; see Figure 4) in which all
things were ranked according to their psyches (roughly translated as “souls”). Rocks and
other inanimate objects had no psyche, and all living things had one or more psyches.
Thus, Aristotle believed in a fundamental dichotomy between life and nonlife 3
Figure 3. Bust of Aristotle.
Among living things, there was a clear hierarchy. Plants had a growing or vegetative
psyche. Animals, too, had a growing psyche, but they also moved. Therefore, they had
an additional or motive psyche. Humans could think, and had a rational psyche in
3
See A Vital Science for an expansion on the vitalist argument.
2
addition to the other two. Categories within the ladder could be further subdivided so
that some were higher and others lower. In such a scheme, designation of mosses as
lower plants made sense, as did humans as highest animals. In a broader sense, this way
of thinking fostered the concept of separate plant and animal kingdoms. Further, it
supported the notion that we, as human beings, were somehow not only higher than the
animals but as different from them as plants were from animals.
MAN
monkey
QUADRAPEDS
flying squirrel
bat
ostrich
BIRDS
amphibious birds
aquatic birds
flying fish
FISH
eels and creeping fish
water serpents
REPTILES
slugs
SHELLFISH
pond mussel
lime-secreting worms
INSECTS
worms
polyp
sensitive plants
PLANTS
trees
shrubs
herbs
lichens
molds
mushrooms
truffles
STONES
stones with layers or fibers
unorganized stones
CRYSTALLINE SALTS
SEMIMETALS
MALLEABLE METALS
PURE EARTH
WATER
AIR
ETHEREAL MATTER
Figure 4. An example of a simplified Scala Naturae from Charles Bonnet (1764).
Aristotle characterized all groups within the Scala Naturae as essentially defined.
That is, they could be defined in such a way that all members have a particular collection
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of characteristics. Such a collection of characteristics then defines or describes the
essence of the group. Consider the following description:
Upright, bipedal primate; nearly hairless, with reduced dentition, opposable
thumbs on forelimbs. Communicates by symbolic language.
This definition, though cumbersome and jargony describes some characteristics that
define the physical essence of our species.
COMMON CONFUSION
Man (homo) is properly derived from Mud (humo). The Greeks called him Anthropos
because, being raised from the dust, he alone among animals looks upward to the
contemplation of his Maker.
-Translation of a Latin Bestiary by T.H. White
Through the Medieval Period and into the Renaissance, essentialistic descriptions had
degenerated into a system of seeking morals from nature. (Reread the paragraph from a
12th Century Latin Bestiary above as a description of a human being). Such descriptions
and definitions became cumbersome, unwieldy, and confusing.
Even today conveying information from one culture to another about a living thing
can be very difficult. Consider how confusing it is now just to make sense of common
names for creatures as familiar as the English Sparrow (see Figure 5). In England, the
same bird is called the House Sparrow. Elsewhere in Europe, the animal is called:
Gorrion (Spain), Musch (Netherlands), Hussparf (Sweden) and Vorobei (Russia). This
difficulty can be overcome by having rules for the unambiguous designation of a
particular organism like the English Sparrow. Now, there are precise rules for taxonomy
(classification) and taxonomic nomenclature (names used in classification). An
International Zoological Congress has jurisdiction over names of animals like the English
Sparrow which is called Passer domesticus, its unambiguous scientific name or Latin
binomial.
Figure 5. A pair of English Sparrows.
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A SWEDE’S KINGDOMS
To acquire knowledge of these things properly, individual specimens should be covered
by a distinct idea and a distinct name, without which the copiousness of things would
necessarily overwhelm us, and, in the absence of a common language, all communication
will cease.
-C. Linnaeus
Carolus Linnaeus (1707-1778; originally called Karl Linne/ and inventor of the Latin
binomial designation; Figure 6) came from Sweden where the English Sparrow is called
Hussparf. He had just returned from an expedition to Lapland, the largely unexplored
northern region of Scandinavia, in 1732. He returned to Uppsala with the expectation
that he would be greeted as a great explorer and scientist. Instead, he discovered that the
Royal Society of Sweden and the University at Uppsala had little to offer him.
Desperate, he set up a museum of the hundreds of specimens of plants and animals that
he had collected and charged admission.
This was the first of a set of disappointments that encouraged Linnaeus to leave
Sweden for a time. In the mean time, he sought to organize his collection for publication,
but found that there were no useful classification systems in general use. This was when
he decided to create a system that was hierarchical and unambiguous.
Figure 6. A portrait of Carolus Linnaeus.
First, Linnaeus organized living things according to essential physical characteristics.
For example, bats have hair and suckle their young, characteristics that make them
5
Mammals. That they have wings and fly (live in the air) was of secondary importance.
He organized flowering plants on the basis of their floral parts as had Andreanus
Cesalpino, regardless of their growth form (herb, shrub, tree).
He organized life within a hierarchical system. First, he recognized that all living
things were either in the Plant or Animal kingdom. Each kingdom had many Phyla
(Phylum is singular) which, in turn, had one or more Classes. There were one or more
Orders within a Class and one or more Families within an Order. The Binomial (Genus
and species) resides within a Family. Each level of the hierarchy had defining
characteristics, and, therefore was essentialistic.
Consider the full hierarchical classification that includes our species:
Kingdom Animalia
Phylum Chordata
Class Mammalia
Order Primates
Family Hominidae
Genus Homo
species sapiens
The Binomial is a particularly important invention as I illustrated with the English
Sparrow example. The first word of the binomial, the genus (genera is the plural), is
capitalized and always is a noun. The second word, the species (species is both singular
and plural), usually is in lower case and is a modifier of the Genus. The English
Sparrow: Passer domesticus, the Human Being: Homo sapiens, the Red Oak: Quercus
rubra, etc. for more than a million described species.
Linnaeus left for Holland where he took a degree in medicine and began to practice.
He also took a manuscript that detailed the system that he had created. This he called
Systema Naturae and found backers for its publication in 1735. It went through many
more editions (see a page from the 10th edition in Figure 7). Although there was some
initial opposition to Linnaeus' system, botanists found it to be quite useful and began to
use it almost immediately. It was less useful for animals and met the greatest resistance
from zoologists.
Systema Naturae (1735 et seq.), and Species Plantarum (1753) form the starting point
for the modern taxonomic system. All scientific names, even binomials published before
1753 for plants and 1758 for animals are not recognized. Curiously, the innovation that
led to the binomials in these books are found in the margins. Linnaeus provided
sentence-long Latin names for the species. However, he annotated them with marginal
trivial names for cataloging purposes. Those trivial names became our specific names
and, coupled with the generic names, have become our useful binomials. It is the trivial
names that have become so important and by which Linnaeus is known.
For Linnaeus, the whole nested system culminating in the Latin Binomial reflected an
underlying order to nature. He was a very strong believer in the fixity of living things
and, therefore, his system reflected order and stability in nature. He fully acknowledged
that because he relied on a few structural characteristics, that his system should be
artificial. That did not matter to Linnaeus. He sought to catalog life by simple and
reliable characteristics. Relationships and "relatedness" had no meaning in a world in
which species did not change.
6
Figure 7. A page from Systema Naturae, 10th edition. The words in the left margins are
the trivial names which became the specific names. In this case, they are species of the
genus Solanum.
DARWIN’S INFLUENCE
Expressions such as that famous one by Linnaeus,…namely, that the characters do not
make the genus, but that the genus gives the characters, seem to imply some deeper bond
is included in our classifications than mere resemblance.
-Charles Darwin
Darwin grasped that the taxonomic hierarchy could be better understood in the
context of descent with modification. One of Darwin’s only illustrations in The Origin
(since straight lines were about all that he could draw) illustrated lines of descent dividing
with some becoming extinct (see Figure 8). The result could be clustered organisms with
various levels of relationships - just like the way that species are clustered in the
taxonomic hierarchy. So, for Darwin and his successors, the way in which living things
7
fit into the taxonomic hierarchy and system of Linnaeus was a confirmation that species
were not fixed.
Considered another way, why were there so many beetles in a specially created
world? Where was the static order? The exuberance of species argued for descent with
modification. Some groups with a particularly successful and malleable form have more
species. Some phyla (plural of phylum) like Ginkgophyta have only a single living
species, Ginkgo biloba. Trees or lines of descent as illustrated in Figure 8 are called
phylogenies. The goal of taxonomy after Darwin was to produce a system that reflected
evolutionary or phylogenetic relationships.
Figure 8. A diagram from Origin of Species that shows how descent with modification
could create patterns of relationships that resemble modern taxonomies.
WHITTAKER’S FIVE KINGDOMS
No part of science is immune to reconsideration, and it may well be that the strength of
the two-kingdom system is more in tradition than in inherent merit.
-R.H. Whittaker
The idea to separate the protists from plants and animals was an old one and
originated with Ernst Haekel, the grand old German master of Darwinism in the latter
part of the 19th century. He proposed a three kingdom framework in which Plants and
Animals were clearly defined. Then, he placed all of the groups like algae, protozoa and
bacteria that did not clearly fit in either kingdom in a third group called the Protista.
Later, proposals were made to separate the Bacteria from the Protista to make a four
8
kingdom system. However, neither Haekel’s nor any other system caught on and the two
kingdom concept persisted until an American, Robert H. Whittaker (1924-1980; see
Figure 9) in 1959 promoted a five-kingdom concept which included the fungi as a
separate kingdom.
Figure 9. Robert H. Whittaker.
Textbooks and universities were slow to come over to this new system. Even as late
as the late 1980’s texts did lip service to the five kingdom system while still teaching the
Plant-Animal dichotomy. This usually was done by dismissing the “animal protists” in
introductory Botany texts and vice versa in Introductory Zoology.
The Five Kingdoms of life were fairly easy to define. Bacteria had no nuclei and
were fundamentally different from other things. For the multicellular kingdoms,
Whittaker returned to Aristotelian-like functional definitions. In particular, he defined
the three multicellular kingdoms based on modes of nutrition. Animals were
heterotrophic (ingested their food), Plants were photosynthetic (made their food by
capturing light energy) and Fungi were saprobic (decomposers that release enzymes to
break down large organic molecules externally and then pull the small molecules
generated in this way across their membranes into their cells). Protists (like the bacteria)
were defined structurally. That is, they were organisms with nuclei that generally were
unicellular in form. Among them occurred organisms that exhibited all three modes of
nutrition.
More recent descriptions such as Five Kingdoms by Lynn Margulis and Karlene
Schwartz attempt to group organisms by structural details at all levels (molecules, cells,
and bodies). This approach is the only way to return to the Darwinian call to classify
organisms based on evolutionary lineages.
9
Figure 10. A comparison between the 2 Kingdom system (above) and Whittaker's 5
Kingdom system (below). From Whittaker's 1959 paper.
SYSTEMATICS
The essence of systematics is evolution.
-Lynn Margulis
The discipline that includes phylogenetic taxonomy is called Systematics. Work in
systematics is much more than the apparent drudgery of cataloguing species, it an
exciting and lively discipline in which discovery and experimentation are necessary
components. It is the science of Biodiversity.
The goals of Systematics according to Simpson and Cracraft (1995) are to answer the
following questions:
• What are the Earth's species?
• What are their properties?
• Where do they occur?
• How are they related?
Certainly, this challenge is daunting. Indeed, right now Biologists have no idea how
many species might exist. The answer could lie anywhere between 2 and 100 million.
So far, we have described about 1 million. Today, the five-kingdom system is challenged
by other multiple kingdom views, some with more than 10 kingdoms. This is more than
a trivial arrangement of taxonomic names. The way in which systematics organizes life
represents a theory about life, its history and interrelationships between living things. In
this sense, classification systems are deductive constructs that serve to explain living
things as well as to organize them.
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HIGHER? LOWER? OTHER?
The concepts of "lower" and "higher" organisms are subject to well-known
ambiguities and various problems …arise in this separation.
-Robert Whittaker
By the time that I left the University of Oklahoma in 1981, dinoflagellates were
neither plants nor animals, but protists. The five kingdoms had come in to general
acceptance by Biologists, and, by 1982, Margulis and Schwartz produced their first
edition of Five Kingdoms. Still, the diagrams looked the same to me. The two kingdom
diagram (Figure 10) was just a double-laddered Scala Naturae. Whittaker's 5-kingdom
diagram was a three-pronged ladder. The concepts of "higher" and "lower" were implicit
in the illustrations. Now the five-kingdom system is being abandoned for a system that
embraces many more kingdoms 4 .
About 1990 I had something of an epiphany, too. I recall reading something that
stressed that all living things had been evolving as long as I had. Put another way, all
other living species are at the ends of their own evolutionary histories. Of course I knew
that rationally, but at that moment the understanding of the statement poured over me like
warm water. In a flash, I had a new and deeper respect for other living things. This
meant that there were no higher or lower organisms, just other organisms. For me, the
Scala Naturae was gone.
-1999, revised 2006
Sources that I used to write the essay:
Adams, Alexander B. 1969. Eternal Quest, The Story of the Great Naturalists. G. P.
Putnam's Sons. New York.
Aristotle (edited and translated by Philip Wheelwright). 1951. Natural Science and
Zoology. Odyssey Press. New York.
Bowler, Peter. 1992. The Environmental Sciences. Fontana Press. London.
Darwin, Charles. 1882. The Origin of Species. 6th edition. D. Appleton and Company.
New York.
Holt, Jack R., Jeffrey R. Merrell, David Seaborn, and Jeffrey Hartranft. 1994. Population
dynamics and substrate selection by three Peridinium species. Journal of Freshwater
Ecology 9(2):117-128.
Margulis, Lynn and Dorion. Sagan. 1986. Microcosmos. Four Billion Years of Microbial
Evolution. A Touchstone Book. Simon and Schuster. New York.
Margulis, Lynn and Karlene Schwartz. 1982, 1992, 1998. Five Kingdoms, An Illustrated
Guide to the Phyla of Life on Earth. Editions 1-3. W.H. Freeman and Co. New York.
Mayr, Ernst. 1982. The Growth of Biological Thought, Diversity, Evolution, and
Inheritance. The Belknap Press of Harvard University Press. Cambridge, Mass.
Mayr, Ernst. 1997. This is Biology, The Science of the Living World. The Belknap Press
of the Harvard University Press. Cambridge, Mass.
Peattie, Donald C. 1936. Green Laurels, The Lives and Achievements of the Great
Naturalists. Simon and Schuster. New York.
Rosenberg, Alexander. 1986. The Structure of Biological Science. Cambridge University
Press. New York.
4
The number of kingdoms has not been settled, but it is many more than 5. Paradoxically, recent
systematic work suggests that Fungi may be allied with the Animal Kingdom. See The Systematics Wars
for an explanation.
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Simpson, Beryl and Joel Cracraft. 1995. Systematics: The Science of Biodiversity.
Bioscience 45(10): 670-672.
Singer, Charles. 1959. A Short History of Scientific Ideas to 1900. Oxford Universtiy
Press. New York.
White, T. H., ed. 1960. The Bestiary, A Book of Beasts. Capricorn Books, G.P. Putnam's
Sons. New York.
Whittaker, Robert H. 1957. The Kingdoms of the Living World. Ecology 38(3): 536-538.
Whittaker. Robert H. 1959. On the Broad Classification of Organisms. Quarterly Review
of Biology 34: 210-226.
Internet Sources
http://www.baylink.org/wpc/esparrow.html
http://www.crl.com/~sarima/dinosaurs/philosophy/
http://www.english.upenn.edu/~jlynch/FrankDemo/People/linnaeus.html
http://www.ilstu.edu/depts/labschl/uhigh/depts/first/classification.htm
http://www.kent.edu/biology/freuden/systemat.htm
http://www.systbot.uu.se/dept/history/linnaeus.html
http://www.thinkwareinc.com/aristo.htm
http://www.ucmp.berkeley.edu/history/linnaeus.html
QUESTIONS TO THINK ABOUT
1. How is a dinoflagellate like a plant? An animal? A Protist?
2. What is the Scala Naturae? What are some vestiges of the Scala Naturae in use
today?
3. What is the concept of essentialism?
4. What is the problem with common names?
5. How did Linnaeus resolve the chaos of classification in the 18th century?
6. Where did the Linnaean Latin binomials really come from?
7. The Linnaean system is hierarchical. How did that organization serve to confirm
Darwin’s concept of descent with modification?
8. How did Ernst Haekel and Robert Whittaker change the landscape of
classification systems?
9. What is systematic biology?
10. Why do Biologists not use terms like higher and lower any more?
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