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, — • 



