238 DIFFICULTIES OF THE THEORY [Chap. VI. 



and taking advantage of all favourable variations, has 

 produced similar organs, as far as function is concerned, 

 in distinct organic beings, which owe none of their 

 structure in common to inheritance from a common pro- 

 genitor. 



Fritz Miiller, 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 Miiller, and which are nearly related, to 

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

 tant characters; namely, in their sense organs, circulat- 

 ing system, in the position of the tufts of hair within 

 their complex stomachs, and lastly in the whole struc- 

 ture 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 be- 

 longing 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 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 im- 

 probable in the highest degree, that their common pro- 

 genitor should have been adapted for breathing air. 



