OF NATURAL SELECTION. 181 



either case could have been developed through the natural 

 selection of successive slight variations; but if this be ad- 

 mitted in the one case it is clearly possible in the other; 

 and fundamental differences of structure in the visual 

 organs of two groups might have been anticipated, in ac- 

 cordance with this view of their manner of formation. 

 As two men have sometimes independently hit on tlie 

 same invention, so in the several foregoing cases it appears 

 that natural selection, working for the good of eacli being, 

 and taking advantage of all favorable variations, has pro- 

 duced 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 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 fev/ species, possessing an air-breathing appara- 

 tus 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 systems, in the position of 

 the tufts of hair within their complex stomachs, and lastly in 

 the whole structure of the water-breathing branchia3, even to 

 the microscopical hooks by which they are cleansed. Hence 

 it might have been expected that in the few species belong- 

 ing 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, 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 

 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 improb- 

 able 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, 



