DIFFICULTIES OF THE THEORY 201 



possible in the other; and fundamental differences of struc- 

 ture in the visual organs of two groups might have been an- 

 ticipated, in accordance with this view of their manner of 

 formation. As two men have sometimes independently hit 

 on the same invention, so in the several foregoing cases it 

 appears that natural selection, working for the good of each 

 being, 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 progenitor. 



Fritz jMiiller, 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 

 important 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 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 Miiller argues that this close similarity in so many 

 points of structure must, in accordance with the views ad- 

 vanced by me, be accounted for by inheritance from a com- 

 mon progenitor. But as that 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 



