182 DIFFIOULTIES OF THE THEORY 



in the manner in which they are opened and closed, and in 

 some accessory details. Now such differences are intelligi- 

 ble, and might even have been expected, on the supposition 

 that species belonging to distinct families had slowly be- 

 come adapted to live more and more out of water, and to 

 breathe the air. For these species, from belonging to dis- 

 tinct 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 sejiarate acts of creation the whole case remains unintel- 

 ligible. This line of argument seems to have had great 

 weight in leading Fritz Miiller to accept the views main- 

 tained by me in this volume. . 



Another distinguished zoologist, the late Professor Cla- 

 parSde, has argued in the same manner, and has arrived at 

 the same result. lie shows that there are parasitic mites 

 (Acaridse), belonging to distinct sub-families and families, 

 which are furnishes 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 tlie modification of the 

 fore legs, of the hind legs, of the maxillse or lips, and 

 of appendages 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 

 remotely allied, by organs in appearance, though not in 

 development, closely similar. On the other hand, it is a 

 common rule throughout nature that the same end should 

 be gained, even sometimes in the case of closely related be- 

 ings, by the most diversified means. How differently con- 

 structed is the feathered wing of a bird and the membrane- 

 covered wing of a bat; and still more so the four wings of a 

 butterfly, the two wings of a fly, and the two wings with 

 the elytra of a beetle. Bivalve shells are made to open and 



