Chap. vi. of Natural Selection. 153 



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 suppo- 

 sition 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, w T ould 

 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 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 

 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 (Acarida?), 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 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 beings, by the most diversified means. How differently- 



