I.

Planarian worms are perfect regenerators. That is, they’re immortal. If you cut off a chunk of one, it will grow back, and the worm will continue as before.

This also means that a large enough chunk will grow into a separate worm. And in fact, some species of planaria reproduce this way, instead of sexually. They split in half, and each half becomes a full worm again.

So, a question: if Wormy the planarian splits in half and becomes two new worms, which one is Wormy?

You can’t just pick one, since they share equal amounts of flesh with Wormy. You could argue that Wormy is dead and that these are his children. But what if only one of the worms survived the process? Wouldn’t it seem odd to say he wasn’t Wormy? Or, a similar case: does a salamander who loses and regrows her tail become her own child? No.

If Wormy isn’t simply dead, and neither of the new worms is the true Wormy, we’re left with only one possibility: both worms are Wormy.

II.

This seems reasonable, at first glance. Wormy is the name of a continuous process, which starts as a single worm and becomes two.

But what about Wormy’s predecessor? Let’s call him Wormy Prime; the little guy who split in half to produce Wormy and some other forgotten twin. Is he also part of that process? The answer can only be yes.

Now we’ve created a monster. Because the same applies to Wormy Prime’s predecessor, and the eventual successors of the newly-split Wormy. By our prior logic, Wormy is the name of a continuous process, which once was a single worm and now is many millions.

Okay, fine, let’s bite this bullet: Wormy is the name for this entire species of planaria. He’s a single entity who happens to have many spatially distinct parts. Similar to the way our original one-worm Wormy was composed of many cells.

III.

But what about other species of planaria? At some point, they had a common ancestor with Wormy. For the asexual varieties, whatever distinctions we see as dividing them happened as a result of random mutations during this splitting/cloning process. Are they not Wormy?

This would seem somewhat arbitrary. If one of Wormy’s halves were to be struck by radiation and slightly mutated while regrowing, we wouldn’t say that he suddenly becomes severed from his Wormy-ness.

Similarly, we can return to our salamander friend: if she were mutated by radiation while regrowing her tail, we wouldn’t say she becomes a new salamander...

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