How can I diagram the main argument and the sub-arguments of a piece like this? Look around today and you can see for yourself that most of the...

Often, you can get a good idea of an author's argument by picking out where they begin and end. As in an essay, a paragraph argument will have an introduction, supporting points, and conclusion. In an essay, the "main argument" is called a thesis. In this example, we're looking for the thesis of the paragraph first and the supporting points (sub-arguments) second.

Main argument

To diagram the main argument of this paragraph, take a look at the first and last sentences.

• Look around today and you can see for yourself that most of the organisms you come across are not making it into the fossil record.


• Thus, it is not at all surprising that there are “missing links” in the fossil record, and this is not good evidence against evolutionary transmutation. 

The author is saying that the fossil record is not a complete picture of the evolutionary history of organisms. (We'll call the author and people who agree with the author group 1.) Based on this paragraph, the author is arguing against another group of people (group 2) who say the opposite: that the fossil record should show a complete history of evolution if transmutation is real. A critical part of this paragraph is that the author wants to explain why the people in this second group are misguided--they are making a poor assumption about the nature of the fossil record.

Group 2 made the first argument. IF transmutation is real, THEN we should see it in the fossil record.

Group 1 argued that group 2's logic is flawed--since we should not expect to see a full evolutionary history in the fossil record at all, the premise "transmutation is real" can stand with or without the qualifying statement.

Sub-arguments

The sub-arguments mirror body paragraphs in an essay. The sentences that come after the intro but before the conclusion are all possible sub-arguments. We're looking for the sentences that best support the main argument. Let's look at each one:

• It takes a rather special combination of physical factors—usually those of swamps or estuaries where remains can be buried in sediment, be compacted and, if lucky, remain undisturbed for millions of years—for the bones or imprints of an organism to achieve a measure of immortality in stone (2).

This sentence is describing how unrealistic group 2's assumption is. The author is saying it's silly to assume that the fossil record is a complete picture of every organism's evolutionary history because preservation of any record is so rare! In fact, this author is saying that fossilization is so complicated that most of the time it doesn't occur. Does that mean an organism didn't exist? No, of course not. What it means is that it's not in the fossil record.

• To then become part of the scientific body of evidence, they have to erode in such a way as not to be destroyed and then found by someone who recognizes their importance (3).

This next argument points out that, even if an organism is preserved in the fossil record, we might not know it. In fact, there are sure to be fossilized organisms that we haven't discovered. Does this mean those organisms don't exist? No, of course not. What it means is that we haven't seen them yet.

• Furthermore, from what we know of evolutionary mechanisms, speciation events are likely to occur in isolated populations (4), and competition will quickly eliminate the less fit of closely similar forms (5).

This next argument points out that even if organisms are fossilized AND even if we find them in the fossil record, we might not understand how they relate to one another. The author points out that organism A might not lead in a straight path from A to B to C to D and so on... In fact, organism A might lead to organism B and C at the same time and organism B might lead to D, whereas C leads to E. In other words, the web of evolution is not a straight line.

• Both processes make it even more unlikely that there will be a smooth, continuous fossil record of intermediaries (6).

Finally, this sentence (not a sub-argument) re-caps the previous points to transition the reader into the conclusion. This sentence serves to say, "Now that you know these things, you can understand why I'm saying what I'm saying next."