THE STRUCTURE AND HABITS OF ANIMALS 39 



small force may very easily cause a very con- 

 siderable rearrangement. Turning to psychology, 

 we see that the tendency of modern research is 

 to show that whatever we do or think is merely 

 the resultant of what we have done or thought 

 or suffered previously. 



It is not difficult to show that, even in our own 

 worldly transactions, changes of environment often 

 produce not only direct and immediate changes 

 and readjustments, but also definite and calculable 

 ones. 



Thus, as an example, let a be a merchant, and 

 b his purse : the combination ab will at once strike 

 you as a natural and stable one. Indeed there 

 is something about the word merchant that seems 

 to suggest inevitably a well-lined purse. Now 

 'et c be a highwayman, and d his pistol : the 

 combination cd is again recognised as a natural 

 and stable one. Now bring the compound ab 

 into the presence of the compound cd, and mark 

 how the stability of the former is shaken : ab, a 

 previously stable compound, becomes at once 

 curiously unstable ; the merchant and the purse 

 that we were wont to view as inseparable are 

 split asunder at once, and the several elements 

 become rearranged in a manner that finds perfect 

 expression in the formula : 



ab + cd = a + bed. 



Again to take another instance which will remind 

 us of the operation known in chemistry as double 

 decomposition : Let a be a small boy, b a penny 

 in his pocket, c an old woman with an apple stall, 



