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Life and Letters of Charles Darwin, volume 2 includes the following letter from Darwin to Charles Lyell, written about a review of his new book:

P.S.—I must tell you one little fact which has pleased me. You may remember that I adduce electrical organs of fish as one of the greatest difficulties which have occurred to me, and — notices the passage in a singularly disingenuous spirit. Well, McDonnell, of Dublin (a first-rate man), writes to me that he felt the difficulty of the whole case as overwhelming against me. Not only are the fishes which have electric organs very remote in scale, but the organ is near the head in some, and near the tail in others, and supplied by wholly different nerves. It seems impossible that there could be any transition.

I can't get the whole meaning of this bolded phrase, though I know the meaning of "remote" and "scale".

Does it mean that the small and thin plates that protect the fish skin are very far from each other?

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I think this means that the different fish which have electric organs are very remote from each other in the size (scale) of their bodies. It doesn't mean scales as in the plates that cover a fish's body: it means scale in the third sense here, the size or level of something.

This makes sense in context, because as far as I understand, in this paragraph Darwin is discussing how electric organs appear in different ways in unrelated fish, which at the time was used as evidence against his theory of evolution, but which would now be considered a case of convergent evolution. Among the different fish which have electric organs, those organs appear in different parts of the body, supplied by different nerves, and the fish themselves are of different sizes. This shows that those electric organs didn't all evolve in the same way from a single ancestor, but it doesn't (as some claimed at that time) disprove the entire theory of evolution.

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I would argue that it actually refers to taxonomic scale. First here is a list of electric fish: Wikipedia. If you click through them, you can see there is a ton of variation in what they look like. While size (which the other answer suggests) could be one piece of information used to classify different species taxonomically, there's clearly a lot of other relevant physical characteristics distinguishing them.

Now think about other examples that Darwin observed in the context of natural selection, namely Darwin's Finches. With the finches, he was looking at ~18 species all in the same taxonomic family. Depending on the species of electric fish known at the time, he was probably looking at species of electric fish from multiple taxonomic families. So with the finches, he had lots of subtle differences between species (at least, subtle relative to the differences between electric fish), which makes it easier to deduce the role that natural selection might have on their evolution.

With the electric fish, fitting the differences observed into the framework of natural selection (at Darwin's time) is much more difficult. As the other answer points out, a major part of the reason for this difficulty is convergent evolution. If it wasn't for the shared and very unique characteristic of electric organs drawing attention to them, some of these species probably wouldn't have even been compared against each other in this way.

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  • Hi, and welcome. Your point is clearly made, but this is an unusual use of the word "scale" to me. Was it common at the time of Darwin? Perhaps you could share some other example use.
    – Adam Burke
    Aug 2 at 1:02
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    I do not find this answer persuasive. While is is surely true that many characteristics had to be considered to determine the taxonomic placement of a species, i do not think "scale" was used to rever to taxonomic placement. When I see "scale" used in works from Darwin's time and a little later, It pretty much always refers to size, particularly in On Growth and Form by Darcy W. Thompson, which discusses this concept often and in detail. So I do not find this answer persuasive. Can you cite any texts from Darwin's general period that use "scale" in the sense of taxonomy? Aug 2 at 2:19

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