32 CATEGORIES OF CLASSIFICATION. 
confirms this result; and I shall attempt to 
show that there are really four, and only four, 
such structional ideas at the foundation of the 
animal kingdom, and that all animals are in- 
cluded under one or another of them. But it 
does not follow, that, because we have arrived 
at a sound principle, we are therefore unerring 
in our practice. From ignorance we may mis- 
place animals, and include them under the 
wrong division. This is a mistake, however, 
which a better insight into their organization 
rectifies ; and experience constantly proves, that, 
whenever the structure of an animal is perfectly 
understood, there is no hesitation as to the head 
under which it belongs. We may consequently 
test the merits of these four primary groups on 
the evidence furnished by investigation. 
It has already been seen that these plans may 
be presented in the most abstfact manner with- 
out any reference to special animals. Radiation 
expresses in one word the idea on which the 
lowest of these types is based. In Radiates we 
have no prominent bilateral symmetry, such as 
exists in all other animals, but an all-sided 
symmetry, in which there is no right and left, 
no anterior and posterior extremity, no above 
and below. It is true that in some of them 
there are indications of that bilateral symmetry 
which becomes a law in the higher animals; but 
