Fox-terriers are born with about four times as much original sin in them as other dogs are...

Jerome’s quip uses theological language (“original sin”) to comic effect, attributing an innate mischievousness to fox-terriers. The humor depends on exaggeration (“about four times as much”) and on the contrast between a solemn moral concept and the everyday experience of living with a high-energy, willful breed. Read more broadly, it reflects Jerome’s characteristic light satire of domestic life: animals (and people) come with temperaments that resist tidy moralizing, and our attempts to judge them often say as much about human expectations as about the creature itself. The line also captures a late-Victorian/Edwardian fondness for witty, anthropomorphic observations about pets.