180 DIFFICULTIES OF THE THEORY Ciur. VI. 



1)0 iilcMitically the saiiie, yet some fundamental difference be- 

 tween them can always, or almost always, be detected. I am 

 inclined to believe that, in the same manner as two inen have 

 sometimes independently hit on the same invention, so natural 

 selection, working for tlie good of each being and taking ad- 

 vantage of analogous variations, has sometimes modified in 

 nearly the same way two organs in two distinct organic beings, 

 which owe but little of tlieir structure in common to inheritance 

 from a common progenitor. 



Fritz MllUcr, in a remarkable work recently published, has 

 investigated a nearly parallel case, in order to test the views 

 advanced in this volume. Several families of crustaceans in- 

 clude a few species which possess an air-breathing apparatus, 

 and are fitted to live out of the water. In two of these fam- 

 ilies, 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-or- 

 gans, circulating system, in the position of the tufts of hair 

 with whicli their complex stomachs arc lined, and lastly in the 

 v/hole structure of the Avater-breatliing Ijranchia^, even to the 

 microscopical hooks by Avhich they arc cleansed. Hence it 

 might have been expected that the equally important air- 

 breathing apparatus would have been the same in the fevv^ 

 species in both families Avhich live on the land ; and this might 

 Iiavc been tlie more confidently expected by those wlio believe 

 in distinct creations ; for why sliould this one apparatus, given 

 for the same special piu^ose to these species, have been made 

 to differ, Avhile all the other important organs are closely simi- 

 lar or rather identical ? 



Fritz Mllller 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 common 

 progenitor. But as the vast majority of the species in tlie 

 above two families, as well as most crustaceans of all orders, 

 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 in each he found 

 it to diffor 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 differences are intelli- 

 gible, and might even have been anticipated, on the siip]iositi<-)n 

 that species belonging to distinct families had slowly become 



