8o EVOLUTION AND NATURAL THEOLOGY. 



chlorophy], whereas roots and tubers exposed to 

 the air, come to resemble stems and leaves, fre- 

 quently developing chlorophyl (as may often be 

 seen in an uncovered potato) and assuming the 

 functions of those portions of the plant. Even 

 in the highest animals, the functions of different 

 organs are interchangeable to a limited extent. 



Again, unity of structure pervades whole 

 Classes, and yet is not absolutely invariable, as 

 might be expected to be the case, if, as is 

 frequently asserted, unity of design were the 

 chief object and meaning of these resemblances. 

 Throughout the great majority of the Sub- 

 kingdom Annulosa, we find a fixed number 

 of somites, whether the body is long or short. 

 The dragon-fly and the crane-fly have no more 

 and the bee and house-fly no less. Huxley 

 ("Anatomy of Invertebrated Animals," p. 398) 

 fixes the normal number of somites in the 

 Insecta and in the higher Crustacea and 

 Arachnida at twenty. 



Again, throughout the Amphibia, Vertebrata, 

 and (with some fossil exceptions) also the Eep- 

 tilia, we find four limbs, normally possessing 

 five digits ; and whether the limbs are used for 

 running, leaping, digging, swimming, flying, or 



