Some circumstantial evidence is very strong, as when you find a trout in the milk. - What?

I have heard this quotation a number of times over the years and taken it at face value but with a nagging question in my mind about the context. The quote is from Henry David Thoreau,

Some circumstantial evidence is very strong, as when you find a trout in the milk.

Yes, I get that a trout in your milk is strong evidence of something, but what? What were the circumstances that actually made this make sense?

The observation was written in 1849 at the time of a dairyman's strike. Apparently there was a rumor making the rounds that the dairymen were diluting their milk with water. Hence, the trout in the milk as compelling evidence in support of a rumor. Now that makes sense to me.

Solving life's little cultural questions, one at a time.