" Indirect inferences about causation can sometimes be drawn from the absence of variations in a comparative analysis. The lack of variation in certain features against a background of system-level change can suggest that these features are constrained or subject to strong evolutionary pressure against deviation. These are evolution's 'dogs that didn't bark,' immortalized in Sir Arthur Conan Doyle's Sherlock Holmes memoir Silver Blaze:
Gregory: Is there any other point to which you would wish to draw my attention?
Holmes: To the curious incident of the dog in the night-time.
Gregory: The dog did nothing in the night time.
Holmes: That was the curious incident. "

― , The Origin and Nature of Life on Earth: The Emergence of the Fourth Geosphere