Classification, or Putting things into groups. In the seventeenth century naturalists began trying to name living organisms, plants and animals, in some systematic way. Common names were fine, but often vague, confusing, changed from one region to the next and the same creature could have more than one name depending on who was doing the naming.

A different system was needed.

That different system was invented by a Swedish botanist Carolus Linnaeus. He realized that all objects could be grouped (note: in principle the system of classification devised by Linnaeus could be applied to stamps, coins, chairs, cars, or almost anything, but we will restrict our discussion to living organisms).

For example, it was obvious to Linnaeus that "all living organisms" represented one large group of objects, which could then be divided into two smaller groups; "plants" and "animals".

This way of doing things seems just common sense and remarkably easy, but it contains two important concepts:

• defining the groups (i.e. "plants" and "animals"), and

• deciding what factors determine where a particular organism is placed (i.e. "all plants are green", and "no animal is green")

Taxonomy, is the process of devising and defining the various groups. In Linnaeus' case, deciding that he needed two groups which he called 'Kingdoms', the "Animal Kingdom" and the "Plant Kingdom".

This is not as easy as it looks. Although it seems obvious at the level of multicellular organisms that some are green (plants) and some move around (animals), at the microscopic level some organisms are both green and move around! Where to they go?

Linnaeus took the idea of groups even further. After he had divided all organisms into two groups (the 'Kingdoms') he took each Kingdom and divided it up even further into smaller groups (which he called "Phyla"). All animals (those in the "Animal Kingdom"), could be divided up into those animals with a stiff rod up their backs (Chordata), and those that didn't (such as the sponges).

Now he had one set of groups nested inside a larger group, and he continued subdividing each smaller group into even smaller and smaller groups, until there was only one creature that would fit into the final, smallest category. At this point he had to stop.

Defining these groups unambiguously started to become a problem for Linnaeus, and it has been a problem ever since. When defining the groups do you either,

• try to make the groups as large as possible (lumping as many organisms together as you can), or,

• split the groups up into many small categories (so as to make each group as unique as possible).

The taxonomic "wars" between the 'lumpers' and the 'splitters' continues to this day.

Once the groups have been established and the criteria for membership defined, it then becomes possible to take any creature and ask a series of questions. The answers to these questions then automatically places that creature into the next subgroup, where the questions start again. For example,

Humans -
First question - **Plant or Animal?** - answer, humans are multicellular, heterotrophic creatures, so, they belong in the "Animal Kingdom".

Second question - **Stiff rod in their back?** - answer, yes, so they belong in the "Chordata" phylum (the name given by Linnaeus to this level of subgroup).

Third question - **Spinal chord surrounded by bone?** - answer, yes, so they belong in the "Vertebrata" subphylum.

And so on.

Today you could go on subdividing into the "Class", then the "Order", then the "Family", then the "Genus" and finally into the "Species". When you finally arrive at these last two groups (the Genus and the Species) you can go no further, so the system stops here.

Nomenclature In the system devised by Linnaeus, the last two names used for each creature, the Genus name and the Species name, are put together to form the scientific name of that organism. In the case of humans the Genus name is "Homo" and the Species name is "sapiens", so these two are put together and the scientific name for humans becomes Homo sapiens. (Note: italics are used for these names and the Genus name is capitalized).

Because each creature is given a "two word name", this is often called the binomial system. Since the time of Linnaeus this system has been expanded and improved, but the controversy concerning the taxa (groups) has not gone away. Linnaeus only had two "Kingdoms", today most people use a system based on five Kingdoms, while some prefer four and others, six. No one system is 'right' or 'wrong', just different.

Classification. Any system of classification (or 'grouping') tries to put objects into the same group if they share characteristics in common, and tries to put different objects into different groups if the differ from one another. Sounds easy? Try it some time!