have lost that button."

"It was an accident I suppose, and one against which he could not have
guarded. It was one of the exigencies of his crime."

"Exactly so; and it is these little accidents, always unforeseen, though
always occurring, which hang so many, and jail so many, and give our
detectives such an easy road to fame. That is the gist of the whole
matter. It is an unequal game, this between the criminal and the
detective."

"I don't catch what you are driving at?"

"I'll give you a dissertation on crime. Attend! In ordinary business it
is brains _versus_ brains. The professional man contends with his
fellows, and if he would win the race towards fortune he must show more
brains. The commercial man competes with other tradesmen all as clever
as himself. So it goes from the lawyer to the locksmith, from the
preacher to the sign painter. It is brains rubbing against brains, and
we get the most polished thought as the result. Thus the science of
honest living progresses."

"What has this to do with the criminal class?"

"One moment. Let the philosopher teach you in his own way. With the
criminal it is different. He is matched against his superior. Those in
his own class do not contend with him; they are rather his partners, his
'pals,' as they term it. His only contention therefore is with the
detective who represents society and the law. No man, I suppose, is a
criminal from choice, and it is the criminal's necessity which leads to
his detection."

"Then all criminals should be caught."

"All criminals should be caught. That they are not is a strong argument
against your detective; for every criminal, we may say is actuated by
necessity, and therein lies the possibility of his defeat. For example:
You may claim that the expert burglar lays his plans in advance, and
that the crime being premeditated he should be able to make such careful
pre-arrangements that he could avoid leaving tell-tale marks behind him.
This, however, is rarely the case, for this reason: the unexpected
often, if not always, happens, and for that he has not prepared. In a
moment he sees prison ahead of him, and his fear steals away his
caution, so that, as we have seen, he does leave a clue behind him."

"But when you say the unexpected happens, you admit the possibility for
that to occur which could not have been premised, and therefore could
not have been guarded against."

"That is true as the case stands. But remove the neces