It is simply this.

Vitality is an exhaustible quality.

It may last up to a certain moment, then it burns out like the hungry wick of a candle that has no more grease to feed it.

You can incarcerate a man for such a length of time that when at last you do give him his liberty he has no love left for it.

It is much the same with these creatures who are imprisoned in the barred cells of London offices.