96 EVOLUTION AND NATURAL THEOLOGY. 



directions, and through continually increasing 

 degrees of complexity, to the highest plants 

 and animals which the earth has yet produced. 



It is an established law in Zoology that 

 animals within the same Class resemble each 

 other more in embryo, or in their young stages, 

 than in their adult forms, and that an embryo 

 passes through a series of developments which 

 are at first undistinguishable from those of other 

 animals, and it only becomes gradually differen- 

 tiated from them as its special characteristics 

 successively appear. The differences between 

 the members of any one group essentially 

 consist in the relative development of the 

 elements which all possess in common.* 



Since the time of Goethe, it has been 

 generally known that nearly all portions of 

 a plant are simply modifications of the leaves. 

 This was, however, first put forward by Wolff 

 in 1759. Many of the dermal appendages of 

 animals, including even the horn of the 

 rhinoceros, are nothing more than modified 

 hairs. 



It was mentioned in the last chapter that the 



* Compare Carpenter's " Comparative Physiology," ed. 4, p. 101. 



