A PHYSICAL ANALOGY 137 



The first position of the model, resting upon the side 

 A B, 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 B C. 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. A strong 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 



