Chap. VI.] OF NATURAL SELECTION. 239 



Miiller was thus led carefully to examine the apparatus 

 in the air-breathing species; and he found it to difEer 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 

 differences are intelligible, and might even have been 

 expected, on the supposition that species belonging to 

 distinct families had slowly become adapted to live 

 more and more out of water, and to breathe the air. 

 for these species, from belonging to distinct families, 

 would have difEered to a certain extent, and in ac- 

 cordance with the principle that the nature of each 

 variation depends on two factors, viz., the nature of 

 the organism and that of the surrounding conditions, 

 their variability assuredly would not have been exactly 

 the same. Consequently natural selection would have 

 had different materials or variations to work on, in 

 order to arrive at the same functional result; and the 

 structures thus acquired would almost necessarily have 

 differed. On the hypothesis of separate acts of creation 

 the whole case remains unintelligible. This line of 

 argument seems to have had great weight in leading 

 Fritz Miiller to accept the views maintained by me in 

 this volume. 



Another distinguished zoologist, the late Professor 

 Clapar^de, has argued in the same manner, and has 

 arrived at the same result. He shows that there are 

 parasitic mites (Acaridae), belonging to distinct sub- 

 families and families, which are furnished with hair- 

 claspers. These organs must have been independent- 

 ly developed, as they could not have been inherited 

 from a common progenitor; and in the several groups 

 they are formed by the modification of the fore-legs,^ 



