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Explain xkcd: It's 'cause you're dumb.
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Nature abhors a vacuum and also anything that's not a crab.
Title text: Nature abhors a vacuum and also anything that's not a crab.


Ambox notice.png This explanation may be incomplete or incorrect: Created by a VACUUM CRAB. Please mention here why this explanation isn't complete. Do NOT delete this tag too soon.

As Megan is telling Cueball, separate species of animals have evolved into "crab-like" forms at different times. Naturalists who noticed the tendency gave it the name carcinization.

It is a specific form of convergent evolution, where differing families of animals (in this case, nominally across the crustacea) develop a tendency towards developing a 'crab' bodyplan to a greater degree than their origins would suggest. A similar process has created several varieties of river dolphin with similar adaptations to their environments, despite being 'stranded' offshoots of different forerunner pelagic species.

"True crabs" (Brachyura) form just a small subset of the Crustacea subphylum, and the Cancer genus is a subset of that, yet there appears to be something about the bodyplan and even resulting behaviour that has meant a number of species have arisen from alternate areas of the family tree that are now trivially indistinguishable without extensive study.

Carl Linnaeus even initially included all Crustacea under the 'Cancer' genus (using the Latin name for crabs), and his taxonomic classification has been heavily refined as further knowledge has come to light, in order to reveal this phenomenon.

Apparently this principle is much stronger in the webcomic than in real life, as shortly after being told this, Megan notices that Cueball (not a crustacean!) has himself turned into a crab. This isn't really evolution as we know it (outside of Pokémon at least), which refers to changes (usually gradual changes, but not always) in a species across generations caused by random mutations. The organisms individually never change, they are merely different from their ancestors, and the organisms with changes that make them more fit for their environment are the ones who are more likely to survive long enough to pass down those changes. What happens to Cueball is more like a transformation, but it could still be called 'carcinization', since he becomes crab-like.

Cueball's sudden transformation is perhaps explained by the title text, that "Nature abhors a vacuum and anything that's not a crab". The text is a reference to Aristotle's Horror vacui, a statement about how empty space tends to be immediately refilled by surrounding things, so vacuums seem to be impossible to maintain. As does "not being a crab", it seems.


[Cueball and Megan are walking next to each other towards the left with Megan looking back at Cueball.]
Megan: Have you heard of "carcinization"?
Megan: The tendency of nature to evolve things into crabs.
Cueball: Oh?
[They continue to walk, both of them looking forward.]
Megan: Yeah, the basic "crab" design has evolved separately a number of times.
Cueball: Evolution just loves making crabs, I guess!
[Narrower panel with only Megan seen walking on while lifting both her arms slightly to each side.]
Megan: Apparently!
[Two smaller beat panels are drawn between the previous and next normal sized panels. The first is a bit lower than the top of the normal panels, and is partly lying in over the other small panel, which is below and to the right of the first. The top panel shows Megan continuing to walk along. The second shows Megan stopping and turning to look back.]
[Megan has turned completely and is looking down at a small crab scuttling along on the ground where Cueball was before.]
Megan: Oh no


In January 2020, less than half a year before this comic was releases, the Dinosaur Comic also released a comic about Carcinization. This web comic is on Randall's list of Comics I enjoy and was also used in 145: Parody Week: Dinosaur Comics. Another popular webcomic, Questionable Content, had a crab-themed comic the day before this comic was released.

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