24. LAW OF DIFFERENTIAL EVOLUTION 

Geographic and ecological barriers favor independent evolution but the divergence of vicariant pairs is not necessarily proportionate to the gravity of the barrier or the duration of isolation. 

Creatures that become separated 
When some barrier is interpolated
Often grow differentiated.

But the gap created
Isn't always directly related
To how long they stay thusly isolated; 
Nor is it aggravated,
When the barrier can't be obviated.