Woof. This is a bear of a sentence, because it's about the grammatical case, which is an extremely uncommon use of the word, so it doesn't parse easily. It doesn't help that it's written in hundred-year-old English.
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CSnyder"It" refers to case, i.e. grammatical case, which the author believes cannot be trusted to take care of itself (whatever that means).I think that Fowler is suggesting that we have to think carefully about case - the right case does not appear of its own accord.