CHAP. VI.] OF NATURAL SELECTION. 143 



structure of the water-breathing branchiae, even to the micro- 

 scopical hooks by which they are cleansed. Hence it might have 

 been expected that in the few species belonging to both families 

 which live on the land, the equally-important air-breathing appa- 

 ratus would have been the same ; for why should this one apparatus, 

 given for the same purpose, have been made to differ, whilst all 

 the other important organs were closely similar or rather identical. 



Fritz Miiller 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 Miiller was thus led 

 carefully to 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 sup- 

 position 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 sur- 

 rounding 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 hypo- 

 thesis of separate 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 maintained by me in 

 this volume. 



Another distinguished zoologist, the late Professor Claparede, 

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

 result. He shows that there are parasitic mites (Acaridae), 

 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 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 



