THE PRINCIPLES OF BIOLOGY. 
21 
leaves, they are either entirely or well-nigh wanting. They are 
manifestly intercepting cells. Over the under epidermis of 
the leaf the tissue hardly varies at all. It is a spongy 
parenchyma, with abundant air-spaces. In vertically growing 
leaves the layer of vertical cells (“palisade layer”) may be 
present on both sides, or on neither. There is here, therefore, 
manifest relation of structure and light. Again, the 
elongation of fibres and of the elements of vessels is due, in 
part at least, to the stretching influence of actively growing 
cells in their neighbourhood, and the reality of this factor 
may be recognised by the elements sometimes being even 
torn asunder. As to the origin of the diversity of form of 
free-growing cells, and of the hairs on an epidermis, let us at 
once acknowledge our complete and absolute ignorance ; and 
in conclusion point out that even in those cases where the 
matter appears to our finite knowledge to be perfectly clear, 
we are arguing in the absence of a general factor of vital 
importance—in the absence, viz., of any excepting the feeblest 
glimmerings of knowledge as to the historical conditions of 
life of the plant in question. 
THE PRINCIPLES OF BIOLOGY. 
The Sociological Section of the Birmingham Natural 
History and Microscopical Society completed their study of 
this work on Thursday, December 17tli, 1S86, when Dr. 
Alfred Hill, F.I.C., delivered an exceptionally able and 
interesting address on the final chapters of Part VI., Laws of 
Multiplication, viz., “Multiplication of the human race” and 
“ Human population in the future.” There was an unusually 
large attendance, and a discussion followed, in which the 
President (Mr. W. R. Hughes, F.L.S.), Mr. F. J. Cullis, Mr. 
R. W. Chase, Mr. W. H. France, Miss Dalton, and others 
took part. A hearty vote of thanks was passed to Dr. Hill on 
the conclusion of his address, which occupied upwards of two 
hours in delivery. 
This important work, which forms the second division of 
Mr. Herbert Spencer’s system of “ Synthetic Philosophy,” 
unifies in the great doctrine of Evolution all the leading 
generalisations of Biological science. The work was issued 
by the author in instalments, commencing January, 1863, 
and terminating March, 1867, when it was published in two 
volumes. It was selected by the Sociological Section for 
examination and exposition, in preference to the first division 
of the system —First Principles —as being specially suitable 
