4 INTRODUCTION. 
its support, supplies the means by which the peculiar chemical 
composition, characteristic of each individual being, is preserved; 
at death, on the cessation of this interchange of matter, the organic 
substance passes into solution or putrefaction. Instead of those 
complex combinations of elements, which form the proximate con+ 
stituents of organic bodies, simpler combinations arise, which being 
taken up by the air or the earth, become anew the vital stimulants 
and the nutriment of that vegetable world, on whose existence the 
life of animals is dependent. Thus we perceive here an interchange 
of matter on a large scale, as we do in every organic being on a 
small one: and perishableness becomes the means whereby new lite 
and fresh youth are effused over the whole of nature’. 
Plants and Animals. 
The above may suffice to give a general notion of organised 
bodies. It scarcely requires notice that the term includes plants 
and animals. 
At first sight it seems easy to distinguish an animal from a 
plant: and even the most unskilled person thinks he has a clear 
notion of the difference. Yet it is just his want of knowledge that 
causes the difference to appear so prominent: whilst he overlooks 
the intermediate links, and thinks, for imstance, of a dog and a 
pear-tree. There are two sorts of judgment with conviction. Such 
a judgment may arise either from want of knowledge, or from pro- 
found insight, the result of long and accurate investigation. Who- 
ever seeks after truth must learn to sacrifice the first, even though 
he may never attain to the second. 
Animals are usually considered as more composite and more 
perfect than plants. Yet when we compare the simple substance of 
which Infusory Animals and Polypi are composed with the orderly 
and beautiful structure of the higher plants, we become satisfied 
that this proposition is far from having a general value. 
It is said, plants are rooted in the ground, and by this token 
are sufficiently distinguished from animals. But it is here over- 
looked, that there are free-swimming water-plants, just as there are 
1 «17 opus nature perenni flore rideat.” LINNAEUS. 
