It may even be given as a general rule, that the less any part of the organisation is concerned with special habits, the more important it becomes for classification.

As an instance: Owen, in speaking of the dugong, says, "The generative organs, being those which are most remotely related to the habits and food of an animal, I have always regarded as affording very clear indications of its true affinities.

We are least likely in the modifications of these organs to mistake a merely adaptive for an essential character.