156 THE ORIGIN OF SPECIES 



species agree most closely in all important characters: namely in 

 their sense organs, circulating systems, in the position of the tufts 

 of hair within their complex stomachs, and lastly in the whole 

 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 ap- 

 paratus would have been the same ; for why should this one appa- 

 ratus, given for the same purpose, have been made to differ, while 

 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 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 certain extent, and in accordance with the prin- 

 ciple that the nature of each variation depends on two factors; 

 viz., the nature of the organism and that of the surrounding con- 

 ditions, 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 separate acts of 

 creation the whole case remains unintelligible. This line of argu- 

 ment 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 ; 



