ON NATURAL SELECTION. 171 



have been made to differ, while all the other important organs 

 were closely similar, or rather, identical. 



Fritz Mailer argues that this close similarity in so many 

 points of structure must, in accordance with the views 

 advanced by me, be accounted for by inheritance from a 

 common progenitor. But as the vast majority of the species 

 in the above two families, as well as most other crustaceans, 

 are aquatic in their habits, it is improbable in the highest 

 degree that their common progenitor should have been 

 adapted for breathing air. Muller was thus led carefully tc 

 examine the apparatus in the air-breathing species ; and he 

 found it to differ in each in several important points, as in 

 the position of the orifices, in the manner in which they are 

 opened and closed, and in some accessory details. Now 

 such differences are intelligible, and might even have been 

 expected, on the supposition that species belonging to dis- 

 tinct 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 condi- 

 tions, their variability assuredly would not have been 

 exactly the same. Consequently natural selection would 

 ave had different materials or variations to work on, in 

 Order to arrive at the same functional result; and the struc- 

 tures thus acquired would almost necessarily have differed. 

 On the hypothesis of separate acts of creation the whole 

 case remains unintelligible. This line of argument seems 

 to have had great weight in leading Fritz Muller to accept 

 the views maintained by me in this volume. 



Another distinguished zoologist, the late Professor Cla- 

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

 the same result. He shows that there are parasitic mites 

 (Acaridse), 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 

 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 



