PRINCIPLES OF BIOLOGY. 
129 
instability, its susceptibility to external forces, and the 
number of its kinds. Illustrations are found in tlie fusible 
metals, which melt at a lower temperature than their 
constituents. 
It is established in “ First Principles ” that any aggregate 
of molecules tends always towards equilibrium, and change 
cannot cease till equilibrium is reached. The aggregate acts 
upon the molecules in such a way as to cause them to tend 
towards certain positions; it is to this tendency that we 
apply the much-abused word “ polarity.” Its effect is seen in the 
growth of a crystal and the annealing of glass. If we 
imagine an aggregate of molecules of iron, for instance, 
unacted upon by external forces, they would exhibit a ten¬ 
dency, supposing them free to move, to place themselves 
all “ heads and tails” ; and we see instances of an approach 
to this state in the sometimes disastrous change which a 
wrought-iron bridge suffers when subject to continuous jar, 
and m the magnetisation of a bar of iron by hammering it 
while placed parallel to the terrestrial magnetic axis. 
Now in the case of a growing organism we know how 
much greater and more rapid the changes are at first; the 
rate of change gradually diminishes, life becomes less active, 
the state of equilibrium draws nearer and nearer, and finally, 
when the molecules of the organism in certain parts cease to 
react to the stimulus of external forces, life has ceased, the 
organism is dead. The degree of activity of life is propor¬ 
tional to the susceptibility of the organic molecules, i.e., the 
physiological units, to the action of the environment. “ When 
therefore we see,” as we have seen, “that gamogenesis recurs 
only when growth is decreasing or has come to an end, we 
must say that it recurs only when the organic units are 
approximating to equilibrium—only when their mutual re¬ 
straints prevent them from readily changing their arrange¬ 
ments in obedience to incident forces.” 
We deduce then this result, that the use of gamogenesis 
lies in the necessity of overcoming this tendency to equi¬ 
librium and re-establishing the capacity for active molecular 
change, “ a result which is effected by mixing the slightly - 
different physiological units of slightly-different individuals.” 
The cells which unite have severally nearly reached a condition 
of equilibrium ; this is shown by the fact that, if ununited, they 
are capable of only a little further growth. But though they 
are individually in equilibria, yet, as they are derived from 
more or less unlike parts, the product of their union is not 
so ; in fact, the slight unlikeness which exists is, as is known 
from inorganic chemistry, the very condition which ensures 
