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I'm looking for a real example from history of science, in which one thing is discovered to actually be two different things.

This is a kind of a reversed Twin Earth experiment: Oscar's community is used to call "water" the transparent liquid that can be found in seas and rivers. After a while, it's discovered that the instances that were used to be called "water" are actually instances of two different kinds that have nothing in common but the fact that they look the same to the community members (say, they are discovered to be H2O and XYZ). If most of them are H2O, and XYZ are only a few, it will be said by the community members that they mistakenly took some instances of XYZ to be water, but water is actually H2O (they might also give a new name to XYZ). But what happens if XYZ and H2O are equally distributed? What, then, will be the result of the discovery? I think that in this case they will say that "water" is a rigid designator of "H2O or XYZ". And if the distribution is 49:51? Isn't it a problem to say that for some distributions the designator designates "H2O or XYZ", and for others it designates only "H2O"?

Is there a real example from the history of science for something like that?

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  • Quite often what was one species based on shape (morphology), has turned out to be more, based on genetics..
    – CriglCragl
    Commented Oct 9, 2022 at 14:46
  • You will find many examples in biology, and probably quite a few in chemistry, especially early metallurgy, maybe gemology. Commented Oct 9, 2022 at 16:01
  • @DavidGudeman, Do you have any specific, documented, example of this kind?
    – Amit Hagin
    Commented Oct 10, 2022 at 8:30
  • Also @CriglCragl, any example?
    – Amit Hagin
    Commented Oct 10, 2022 at 8:30
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    An example of what @CriglCragl states is here: Animals That Seem Identical May Be Distinct Species. Researchers examined segmented worms and conducted DNA analyses, discovering that the worms were in fact two different species. However, both species differed in one of the examined genes by 17%, which is twice as much as the equivalent difference between humans and chimpanzees. Commented Oct 10, 2022 at 17:38

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At one time planets were called "wandering stars" until it was discovered they are quite different.

This is discussed on the sister site SE Astronomy at When it was discovered that the classical planet are not stars?

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  • This is an interesting example. Yet, there was an apparent difference between a “star” and a “wandering star”. I’m looking for something that was completely hidden
    – Amit Hagin
    Commented Oct 10, 2022 at 8:32

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