Page - 202 - in The Origin of Species

202 ORIGIN OF SPECIES differences are intelligible, and might even have been ex- pected, 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 cer- tain 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 condi- tions, 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 ar- rive 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 re- mains unintelligible. This line of argument seems to have had great weight in leading Fritz Miiller to accept the views maintained by me in this volume. Another distinguished zoologist, the late Professor Clapa- rede, has argued in the same manner, and has arrived at the same result. He shows that there are parasitic mites (Acaridje), belonging to distinct sub-families and families, which are furnished with hair-claspers. These organs must have been independently developed, as they could not have been inherited from a common progenitor ; and in the several groups they are formed by the modification of the fore-legs, —of the hind-legs,—of the maxillae or lips,—and of append- ages on the under side of the hind part of the body. In the foregoing cases, we see the same end gained and the same function performed, in beings not at all or only re- motely allied, by organs in appearance, though not in de- velopment, closely similar. On the other hand, it is a com- mon rule throughout nature that the same end should be gained, even sometimes in the case of closely-related beings, by the most diversified means. How differently constructed is the feathered wing of a bird and the membrane-covered wing of a bat; and still more so the four wings of a butter- fly, the two wings of a fly, and the two wings with the elytra of a beetle. Bivalve shells are made to open and shut, but on what a number of patterns is the hinge constructed,—•