exceptio probat regulam in casibus non exceptis
—Cicero
I was enlightened by looking at an old textbook about arthropods. The book was divided into three chapters, which sounded like the three branches of the military:
At first I was outraged: But there are aquatic spiders! And land-dwelling crabs! And the termites live underground! But then, of course, aquatic spiders build air bubbles to breathe underwater. And land-dwelling crabs go back to the water to reproduce. And the termites actually fly when spawning. Thus, the classification is not as bad as it seems.
Strictly speaking however, this classification is false. There are legitimately terrestrial crustaceans such as woodlice. And there are insects that do not fly such as earwigs, and even without wings such as silverfish. However, these cases are easy to describe as exceptions of the general rule. This simple table is very useful as a mental framework for understanding the big picture. Besides inaccuracies, the table of arthropods is incomplete; it lacks the myriapods (centipedes and millipedes), and some other groups containing horseshoe crabs, sea spiders and the extinct trilobites.
In the same spirit, I present below a simple classification of all living things. All the entries in this table are incomplete, and all the descriptions are false. However, they give me a satisfying feeling of the main patterns. I wrote this table by looking at many wikipedia pages starting from Biological classification.
I have struggled many years to understand the complexity of the tree of life. Now I see that it is impossible to understand, but that it can be greatly simplified. For example, multicellular lifeforms are mostly of three types, depending on where they get the energy from
Of course, there are carnivorous plants, and even photosynthetic animals. But if you want to speak about them as exceptions, you first have to state the rule.
Here’s the whole table:
The table is outrageously incomplete, and all the descriptions are either stupid or false. For example: