A PHYSICAL ANALOGY 127 
The first position of the model, resting upon the side 
AB, may be taken to represent the condition of a 
type or stable form. A comparatively small push 
(variation) will lead to the production of the subtype 
illustrated by the position BC. When in this new 
position, it is easier to cause the model to return to its 
original position A B than it is to make it pass on to the 
new and more modified position resting upon the side 
CD. Astrong push (mutation) may force the model 
to pass through the position C D until it comes to rest on 
the side opposite to A B. This fresh position represents 
a new stable form, and it is now once more surrounded 
by positions of subordinate stability—subtypes. 
One more analogy before we pass on to consider the 
more recent observations upon discontinuous varia- 
tions or mutations. We may compare the difference 
which exists between deviations and stable forms, 
arising by fluctuating and by definite variation respec- 
tively, with the behaviour of the atoms of chemistry, 
as expressed in the account of their structure recently 
given by Professor Sir J. J. Thomson. Such an atom 
is regarded as being made up of a number of electrons 
or corpuscles bearing definite relations to one another 
in space. In certain circumstances it seems that it 
may be possible to remove a series of these corpuscles 
from the atom one at a time, in which case every such 
successive removal would be accompanied by a com- 
paratively gradual and progressive change in the 
properties of the atom so modified. But after a 
certain time a point would be reached at which the 
removal of one more electron would necessitate a 
