THE PRINCIPLES OF BIOLOGY. 
71 
THE PRINCIPLES OF BIOLOGY. 
BY HERBERT SPENCER. 
EXPOSITION OF CHAPTER II., 
BY ALFRED HILL, M.D., F.I.C. 
The Actions of Forces on Organic Matter . 
The various forces which affect organic matter are heat, 
light, mechanical force, quasi-meclianical force as seen in 
the absorption of water and osmose, chemical affinity, and 
indirect chemical action or catalysis. The increased mole¬ 
cular vibration and consequent assistance rendered to the 
action of the other incident forces which are occasioned by 
the influence of heat are noticeable, and the more direct 
results of heat are exemplified by the evaporation which it 
occasions, one result of which is that circulation is started in 
the tissues of plants and animals, as seen in the withering of 
a plant whose roots are lacking sufficient moisture. The 
effect which light has on mineral, vegetable, and animal forms 
of life is obvious. As evidence of the compound character 
of a ray of light attention is directed to the fact that 
it is the yellow or luminous portion of the ray which 
affords the plant the opportunity of decomposing its 
mineral food and of fixing the hydrogen and carbon in its 
tissues and secretions. A consideration of the undulatory 
theory of the nature of light furnishes an explanation of 
the manner in which these changes are considered to be 
effected. One thing, however, is clear beyond doubt, and 
that is that light is absolutely necessary for the production of 
chlorophyll, the colours of the flower petals, and other similar 
results too numerous to state. The importance of the quasi- 
meclianical forces is shown in the absorption of water, and 
the introduction with it of the agents of chemical change, 
as well as in the conveying away of the products of such 
change. The physical phenomenon termed osmosis is one 
to be explained at length, as well as its instrumentality 
in contributing to the work of redistribution in organised 
bodies. The most important force, however, is chemical 
affinity, the part which oxygen performs in this character 
being very important. Ordinary chemical action must be 
compared with indirect chemical action or catalysis, so 
that we may have a clear conception of the peculiar nature 
