DIFFICULTIES OF THE THEORY 265 



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 im- 

 probable in the highest degree that their common pro- 

 genitor should have been adapted for breathing air. 

 Miiller was thus led carefully to examine the appa- 

 ratus in the air-breathing species; and he found it to 

 differ in each in several important points, as in the po- 

 sition 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 

 principle that the nature of each variation depends on 

 two factors, viz., the nature of the organism aud 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 

 separate acts of creation the whole case remains unintelli- 

 gible. 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 

 Clapar^de, has argued in the same manner, and has ar- 

 rived at the same result. He shows that there are para- 



— Science — 12 



