6 INTRODUCTION. 
tinal canal of insects and worms, can scarcely be devoid of organic 
components. In general it cannot be contested that the vegetable 
kingdom prepares from inorganic nature those substances which 
serve for nutriment to the animal kingdom. Albumen and Fibrin, 
the principal constituents of the blood of animals, are already 
present in the parts of plants which they consume. But still, on the 
other hand, there are plants which grow on other plants, and thus 
apply to their own development the matters which have been pre- 
pared by those organic bodies. 
Scruples arising from such considerations must not mislead us 
to consider plants and animals as belonging to one and the same 
kingdom. Perhaps the following remarks may help to distinguish 
them from each other. 
If we consider the nutrition, we perceive that animals convey 
their food by one or more apertures into a common cavity, the 
stomach or intestinal canal, from which the prepared matters are 
absorbed and applied to the nutrition of the whole body. Thus the 
intestinal canal is for animals what the soil and air are for plants. 
The plant is consequently so constructed that its surface has the 
greatest possible extension: in the animal all is contrived for union 
round a center. Moreover the plant, which receives nutriment by 
means of its surface and the parts there situated, (pores, hairs, &c.) 
has no need to seek for food: it lives in the midst of its food: when 
this is deficient it cannot move and must consequently die. The 
animal, on the contrary, is destined to seek its food, which it must 
conduct into its intestinal canal: it moves therefore when nutriment 
is deficient. Let it not be here objected that plants move towards 
the light, and send larger roots towards the side where moisture is 
more abundant—for this would be to confound growth with motion. 
The stimulants (light, moisture, &c.) act upon the plant, and there- 
fore its growth is more vigorous in that direction. The animal has 
independent motion which is excited by internal stimuli. Hence 
sensation is ascribed to animals. In the higher animals it is known 
that the contraction of the muscles is under the influence of the 
nervous system: that the stimuli, of whatever kind, if they pro- 
duce motion, act upon the nerves and through these upon the 
muscles. Comparative Anatomy, it is true, has, in some animals, 
hitherto failed to demonstrate a nervous system; but it does not 
therefore follow that these animals do not possess sensation, any 
