Now such differences are intelligible, and might even have been expected, on the supposition that species belonging to distinct families had slowly become adapted to live more and more out of water, and to breathe the air.

For these species, from belonging to distinct families, would have differed to a certain extent, and in accordance with the principle that the nature of each variation depends on two factors, viz.

, the nature of the organism and that of the surrounding conditions, their variability assuredly would not have been exactly the same.

Consequently natural selection would have had different materials or variations to work on, in order to arrive at the same functional result; and the structures thus acquired would almost necessarily have differed.

On the hypothesis of separate acts of creation the whole case remains unintelligible.