In O. Henry's view, human character doesn't really change dramatically over the years but only becomes more and more sharply defined. Silky Bob describes Jimmy Wells to the police officer, whom he doesn't recognize as his old friend, as follows:
"But I know Jimmy will meet me here if he's alive, for he always was the truest, stanchest old chap in the world. He'll never forget....He was a kind of plodder, though, good fellow as he was."
In the end it doesn't seem surprising that Jimmy should have become a cop. As a "true" and "stanch" man, the job of upholding law and order would have appealed to him. As a "plodder," he would like the security and routine of poliice work, even though it was poorly paid. Over the years he has become molded more and more firmly into the role he chose, while in the meantime, Silky Bob, who is a restless, ambitious. flamboyant risk-taker, has become more and more firmly molded into the role of a flashy career criminal.
O. Henry characterizes Silky Bob by his appearance:
The man in the doorway struck a match and lit his cigar. The light showed a pale, square-jawed face with keen eyes, and a lititle white scar near his right eyebrow. His scarfpin was a large diamond, oddly set....The waiting man pulled out a handsome watch, the lids of it set with small diamonds.
This describes a successful man of O. Henry's day. He is smoking a cigar and flashing his diamonds. Men did not wear wristwatches in those days. Successful men carried big gold pocket watches in their vest pockets.
Jimmy Wells and Silky Bob were friends twenty years ago, but they have gone off on separate paths and their characters have evolved accordingly. This divergence in people's characters with the passage of time is the principal theme of O. Henry's story. The ways in which they would change were predetermined by their characters when they were young and relatively naive and immature.
Jimmy's action in having his old pal arrested comes as a surprise because O. Henry loved to give his stories surprise endings. In fact, a story with a surprise ending is often described as an "O. Henry story." But the reader does not feel misled. The ending seems to be completely appropriate, given the reader's impression of Jimmy's youthful character as a true, loyal, dependable and "stanch" man who, as Silky Bob says, was "a bit of a plodder" and who loved New York.
"You couldn't have dragged Jimmy out of New York; he thought it was the only place on earth."
Jimmy still had enough of a feeling of friendship for Bob that he couldn't bring himself to make the arrest in person. But he made a point of find out that Bob would be waiting there in the doorway for at least another half-hour. This enabled Jimmy to get another officer to make the arrest. The arresting officer hands Bob a note which includes the sentence:
"Somehow I couldn't do it myself, so I went around and got a plain clothes man to do the job."
Jimmy had developed too strong a sense of duty to overlook the fact that he recognized his old friend and knew he was the man who was wanted in Chicago.
O. Henry must have worried about the ending of this story, although the rest of it might have been easy enough to write. He decided not to have Jimmy arrest his old friend. Such a denouement would have led to a lot of protests and recriminations. It would have left the reader with an unfavorable impression of Jimmy. O. Henry shows good artistic taste in having Jimmy make the arrest by proxy.