Perhaps Faith would understand the analogy better if we were to say that the water flowing into the tub was tinted with different colored dye from time to time and that the porcelain of the tub could be differentially stained because the glaze was worn off to varying degrees. Some dyes are more readily absorbed by different areas of the porcelain (selection), so the pattern of the color of the tub itself (the phenotype) would change over time. The water in the tub at any given moment might be a swirling mix of different colors while the color pattern of the tub itself slowly evolved.
Capt.
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