Steve Shaw
to separate off his good science from his more, er, speculative and whimsical side.

You just aren't getting it are you. In 1700, alchemy wasn't "speculative and whimsical", it was mainstream and conventional. Chemistry as we understand it didn't get started until the second half of the the 18th century with Lavoisier and his contemporaries. Newton was working within the conventions "of his time". Stop trying to expect him to know everything that has happened in the following three hundred years. What do you think he was culpable of?

If you don't agree that evolution is true, the only alternative available to you is "evolution is not true".

Try -
If you don't agree that evolution is purple, the only alternative available to you is "evolution is not purple".

Neither "true" nor "purple" are terms that can be meaningfully applied to evolution.

Does evolution happen? If it does, evolution is true.

Weather happens. Is weather true?

Actually, does evolution happen? Show me where I can see it happening.