882 
THE PRINCIPLES OF BIOLOGY. 
(3).—External forces act unequally on different parts and 
sides of an aggregate. 
Tin's is most markedly manifest in respect to light, but 
holds good equally for all exterior forces, bearing in mind 
that, in addition to the sides, commonly so called, the plant 
likewise has outside and inside. 
In order to obtain the classification of form he hypothe¬ 
cates, Mr. Spencer proceeds to give certain definitions, such 
as asymmetrical for utterly irregular forms, unsymmetrical for 
those approximating to regularity. Of symmetry itself, we 
have that of the sphere, spherical symmetry , as the most 
primitive. He asks you then to consider the gradual flatten¬ 
ing of a sphere to a plane, this latter having radial symmetry. 
Another kind of symmetry appears needed here, that of the 
cylinder, showing equi-radial symmetry in a series of consecutive 
planes. Further, there is bilateral symmetry , which may be 
triple, i.e., divisible into equal and similar parts in three 
planes at right angles, e.y., a brick or shuttle, double, divisible 
in two planes at right angles, e.y., a canoe, and single, divisible 
in but one plane, as in a boat. These terms might be 
conveniently replaced by triaxial, biaxial, and uniaxial 
symmetry respectively. 
The process of evolution would theoretically progress from 
perfect spherical to single bilateral symmetry. This is 
apparently true in fact. 
The simplest plants, amongst aggregates of the first order, 
that is, unicellular organisms, are spherical. The author 
selects Protococcus as his illustration ; the coccus forms of 
bacteria would perhaps provide the best illustration, as, being 
independent of light, and of aquatic habit, they can be sym¬ 
metrical from the point of view of external forces also. 
Spherical symmetry is indeed due to the equality of internal 
and incident forces. The most modern theory of cell-nutrition 
is that the nucleus is, in the ultimate, the feeder. For 
spherical symmetry the nucleus should he central; this it 
apparently is. Directly the nucleus becomes a-central, 
spherical symmetry would be lost, unless counterbalanced 
in some way. Triaxial symmetry exists in diatoms and des- 
mids, and is associated with motility, and triaxial symmetry 
in the arrangement of forces. A cylinder may possibly be 
looked upon as a case of multiaxial symmetry. 
In Caulerpa we have a fixed unicellular organism. In all 
fixed organisms the primary difference is between the free 
and fixed ends, i.e., the disappearance of the spherical and 
tendency to radial or to multiaxial svmmetrv. Another 
illustration can be obtained from the mycelium of a 
