gradual variation

Here we see the head increasing insensibly to the middle, from whence it lessens gradually until it mixes with the neck; the neck loses itself in a larger swell, which continues to the middle of the body, when the whole decreases again to the tail; the tail takes a new direction; but it soon varies its new course; it blends again with the other parts; and the line is perpetually changing, above, below, upon every side. In this description I have before me the idea of a dove; it agrees very well with most of the conditions of beauty. It is smooth and downy; its parts are (to use that expression) melted into one another; you are presented with no sudden protuberance through the whole, and yet the whole is continually changing.

-- Edmund Burke A Philosophical Enquiry into the Sublime and Beautiful