4G 
THE PRINCIPLES OF BIOLOGY. 
already discussed for us by Professor Hillliouse. A similar 
review of animal forms and the mode of their production and 
modification forms the subject of the chapters before us, in 
which it is shown that, in addition to those forces which 
modify the forms of plants, we have an important new factor 
in the locomotion which is so prominent a characteristic of 
animate life. 
It is further shown that in animals which are rooted or 
fixed, and therefore subject to much the same conditions as 
prevail in the vegetable kingdom, we have that well-known 
resemblance to plant forms which the Sea Anemone has made 
familiar to our children, and which is so much more 
strikingly displayed by many of the compound Coelenterata. 
Mr. Spencer reminds us that among the Protozoa, where 
an average all-round equality of conditions obtains, we have 
many instances of spherical form, which may be regarded as 
the Protozoon ideal, from which, without good cause being 
shown, it never departs ; while in animals whose environment 
supplies an average equality of forces about an axis, we find 
a cylindrical or radial symmetry; and where the incident 
forces are equal and opposite on the two sides of a median 
plane, and there only, we get that bilateral symmetry which 
is so constantly and beautifully developed in all higher animal 
forms. 
The radial symmetry of the Echinodermata having been 
justified by the evolution hypothesis, we read (p. 178):— 
“ On watching the ways of the common Sea-urchin, we are 
similarly furnished with an explanation of its spherical, or 
rather its spheroidal figure. Here the habit is not to move 
over any one approximately flat surface ; but the habit is to 
hold on by several surfaces on different sides at the same 
time. Frequenting crevices and the interstices among stones 
and weeds, the Sea-urchin protrudes the suckers arranged in 
meridional bands over its shell, laying hold of objects now on 
this side and now on that, now above and now below; the 
result being that it does not move in all directions over one 
plane, but in all directions through space. Hence, the 
approach in general form towards spherical symmetry—an 
approach which is, however, restrained by the relations of 
the mouth and vent, the conditions not being exactly the 
same at the two poles as at other parts of the surface.” 
Passing on to higher forms, Mr. Spencer, speaking of the 
Annulosa (p. 182), says :—“ The common Earth-worm may be 
instanced as a member of this sub-kingdom that is among the 
least conspicuously bilateral. Though internally its parts 
have a two-sided arrangement, and though the positions of its 
