238 DIFFICULTIES OF THE THEOEY [Chap. VI. 



being, and taking advantage of all favourable variations, 

 lias produced similar organs, as far as function is con- 

 cerned, in distinct organic beings, which, owe none of 

 their structure in common to inheritance from a common 

 progenitor. 



Fritz Muller, in order to test the conclusions arrived 

 at in this volume, has followed out with much care a 

 nearly similar line of argument. Several families of 

 crustaceans include a few species, possessing an air- 

 breathing apparatus and fitted to live out of the water. 

 In two of these families, which were more especially 

 examined by Muller, and which are nearly related to 

 each other, the species agree most closely in all impor- 

 tant characters ; namely in their sense organs, circula- 

 ting system, 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 

 microscopical 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 apparatus 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 Muller 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 im- 

 probable in the highest degree, that their common 

 progenitor should have been adapted for breathing air. 



