264 THE ORIGIN OF SPECIES 



clearly possible in the other; and fundamental differences 

 of structure in the visual organs of two groups might 

 have been anticipated, in accordance with this view of 

 their manner of formation. As two men have sometimes 

 independently hit on the same invention, so in the sev- 

 eral foregoing cases it appears that natural selection, 

 working for the good of each being, and taking advan- 

 tage of all favorable variations, has produced similar or- 

 gans, 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 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 crus- 

 taceans 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, circulating system, in the 

 position of the tufts of hair witbin their complex stom- 

 achs, 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 ex- 

 pected that in the few species belonging to both families 

 which live on the land, the equally-important air-breath- 

 ing apparatus would have been the same; for why should 

 this one apparatus, given for the same purpose, have 

 been made to differ, while 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 



