VISIT TO CHIRK. 
17 
grounds, and Offa’s dyke at a mile distant, but time did not 
permit of a visit to either, though they had been included in 
the programme. Offa’s dyke, which extended from the Dee 
to the Wye, is here to be seen in as almost perfect a state as 
when it was constructed by him in the eighth century to 
defend his country, Mercia, from the incursions of the Welsh. 
After the visit to the castle a return was made to the 
hotel, where tea was served, and the party arrived in 
Birmingham at half-past seven. The comfort of the party 
was greatly enhanced by the courtesy of Mr. Paxton, of the 
Great Western Railway, who provided saloon accommodation, 
and in other ways made special arrangements to promote 
their convenience. The conduct and arrangement of the 
excursion was made by Mr. J. Morley, the honorary secretary. 
THE PRINCIPLES OP BIOLOGY* 
BY HEBBERT SPENCER. 
Exposition of Part IV., Chapters VI. to XII. 
“ The Morphological Composition of Plants.” 
BY W. HILLHOUSE, M.A., F.L.S. 
[Abstract concluded.] 
Spherical symmetry is rare in aggregates of the second 
order, but a case is met with in Volvox. No doubt the 
comparatively perfect symmetry of this plant is due to its 
being free and motile, so that the distribution of forces can 
be equalised. Most of these higher organisms are fixed, and 
spherical symmetry becomes necessarily lost; a tendency to 
it is, however, shown amongst Florideans, while Fuchs shows 
bilaterality or asymmetry. It must be borne in mind that in 
large developments perfect and absolute equality of part is an 
impossibility ; hence many organisms which, like the my¬ 
celium of Mucor , Mr. Spencer tends to consider asymmetrical, 
may be just as symmetrical as, from a finer point of view, 
Protococcus is. Thus to the naked eye view the cap of an 
ordinary Agaric is radially symmetrical, but if you insist 
on mathematical accuracy, probably radial, or any other, 
symmetry does not exist. All these aggregates of the first 
and second orders the botanist speaks of as morphologically 
undifferentiated into members; i.e., as constituting a thallus. 
* Birmingham Natural History and Microscopical Society, Socio¬ 
logical Section, Dec. 17tli, 1885, and Jan. 21st, 1886. 
