INTRODUCTION. 21 



acHeved in proportion to the accuracy of our perceptions of 

 the natural relationship subsisting among the organs thus 

 modified. 



The anterior organs of the different order of vertebrated 

 anifiials, for instance, are organically modified to the degree 

 of their intelligence, their powers of locomotion, and their 

 peculiar habits. In some quadrupeds they are adapted for 

 the prehension of food and for locomotion ; in the bird they 

 are organized for flight ; in the fish, for balancing the body 

 and assisting its progress through the water. The twisted 

 arm of the tortoise can be applied to no other purpose than 

 that of creeping, and the enormous hand of the mole can be 

 used only for burrowing. Yet the anterior members in the 

 different orders of the vertebrata consist essentially of the 

 same parts as those which exist in the same members in 

 man. We find in each the same bones, muscles, nerves, 

 and vessels. Yet how different their appearance ! how 

 varied their functions ! All these ends are attained by a 

 modification in the development of the different parts, one 

 bone being largely developed, a contiguous one less so, 

 some being evolved to a maximum, whilst others are left 

 rudimentary. 



We have a manifestation of the same organic law in the 

 vegetable world. Thus the leaves of plants are variously 

 modified so as to be rendered subservient to the exercise of 

 the different vegetative functions. The different organs 

 appended to the vegetable axis and designated as scales, 

 stipules, bracts, sepals, petals, stamens, and pistils, are only 

 a series of leaves in a state of progressive or retrograde de- 

 velopment, which have assumed this peculiarity of form in 

 consequence of the peculiar and distinct functions assigned 

 them. A fully developed leaf consists of two parts, a little 

 stalk or support called a petiole, and a flat expanded por- 



