12 INTRODUCTION. | 
bear, in their minute structure, such an analogy to each 
other, that we cannot detect those structural differences, 
which are the first clue to the discovery of differences in 
function. Except in the case of the flowers, there is no part 
of the vegetable system in which the adaptation of structure 
to perform a certain function is exhibited with that clearness 
which strikes us in contemplating the system for the circu- 
lation of the blood, that for digestion, for respiration, for 
vision, for motion, for hearing, &c. in animals. Plants are 
much more than animals, of a homogeneous structure. — 
Hence, our knowledge of the physiology of vegetables is very 
imperfect. 
29. A very striking point of distinction between animals 
and vegetables, is found in the character of the nutriment 
by which they are supported. Animals derive their sus- 
tenance solely, from previously organised matter; while — 
vegetables can live and flourish upon inorganic matter alone ae 
and indeed their food may be considered, even when formed 
_ from organic substances, as reduced to an inorganic state 
before being taken in. Animals either live upon vegetables, — 
or upon other animals, whose support has been derived from — 
the vegetable kingdom. Indeed, not only do animals depend - 
upon plants for their immediate subsistence ; but it is by the 
_ growth and decay of suecessive series of plants, from the 
humblest tribes upwards, that a barren rock becomes cover- 
ed with a soil, and that a poor soil is rendered capable of | 
rearing those higher classes of vegetables which give sup- 
port to the animal creation. Vegetables may exist (and 
_ do) where there are no animals, but without the vegetable 
creation the race of animals would soon become | 
