Some students and I, in a special readings class, were working our way through some chapters of David Friedman's law and economics book, Law's Order.
In an interesting section where he makes a persuasive case for allowing a market in babies, he digresses to talk about cats. Here's the sentence:
Rationing goods in excess supply is not usually a problem.
He goes on to point out that that's what the humane society does, even though the alternative to finding so-so homes for cats is killing them. The section can be found on p. 185.
Here's a bonus: another good passage (from page 8):
You live in a state where the most severe criminal punishment is life imprisonment. Someone proposes that since armed robbery is a very serious crime, armed robbers should get a life sentence. A constitutional lawyer asks whether that is consistent with the prohibition on cruel and unusual punishment. A legal philosopher asks whether it is just.
An economist points out that if the punishments for armed robbery and for armed robbery plus murder are the same, the additional punishment for the murder is zero--and asks whether you really want to make it in the interest of robbers to murder their victims.