86 INTRODUCTION. 
limits. The second division is characterized in a much 
less positive manner. It embraces beings of very vari- 
ous structure, possessing but few characteristics in com- 
mon, and created on plans of organization so widely 
different, that nothing brings them together in the same 
group except the negative quality of wanting those 
properties -which distinguish the first. In some of 
them, the nervous system remains undiscovered, and 
if it exists, is less developed than in the ganglionary 
animals. This classification, which is not altogether 
philosophical, is therefore liable to be re-modelled, as 
the progress of knowledge shall require it, and may be 
said to be, at present, in a rather unsettled state. 
Various propositions have been made by different 
naturalists, each influenced by his peculiar theoretical 
views, for its subdivision into natural classes, but none 
of these have met with general acceptation. It is 
certain, however, that the Invertebrata will be recognized 
to contain several distinct divisions, each holding a rank 
equally as independent as the vertebrate animals, dis- 
tinguished by a principle of conformation equally definite, 
and removed to a greater or less distance from them, 
according to the perfection of the structure and func- 
tions of the animals contained in it. 
The order and system impressed by the Divine Will 
on all created things, indicated to the early philosophers, 
as clearly as to those of our own time, that some princi- 
ple of arrangement must prevail in the constitution of 
animated beings ; and hence their attention was given 
