210 THE STRUGGLE FOR LIFE. 
cells are themselves ever going through a constant cycle of 
youth, growth, maturity, and removal from the body, either 
by absorption, or by casting off the whole cell. To prove 
this, I need only mention the case of the hair and nails, 
which are as truly parts of our bodies as are the heart and 
lungs. This change which comes about by a descent from 
cell to cell, no more alters the identity of the body than 
would a journey from here to London. These facts, I 
think, justify my previous assertion, that mysteries exist, 
enshrined within our bodies, likely to-ever remain incom- 
prehensible to merely human faculties, and pushed to their 
logical conclusion they almost justify the school of thinkers, 
headed by Bishop Berkeley, who assert that there is no 
such thing as matter, that it does not exist at all. 
We can, to a certain extent, by the exercise of our wills, 
influence the growth, and increase or decrease, of the cells 
of particular organs of our bodies. But this can only be 
effected in one way, that is, by increasing or decreasing 
the use of that organ. For example, the arm of the black- 
smith, by constant use, increases in bulk and _ strength 
until it becomes capable of feats of power and endurance, 
quite unattainable by the purely brain-worker, and vice- 
versa. It follows from this, that the attainment and 
maintenance of perfect health, the ‘“‘mens sana in corpore 
sano,” or a sound mind in a sound body, depends on a 
due balance in the use of each and every organ of the 
body and mind, so that no undue development or wasting 
shall take place in any of them. Unfortunately this con- 
dition cannot be maintained for more than a limited time, 
since old age must come, and some structures are more 
highly vitalised or stronger than others from the beginning, 
so that they grow old more rapidly, in the one case one 
tissue or organ being the weaker, in the second another; 
and so on. 
I have now to enter on the second branch of my subject, 
