This section is from the book "The Dogs Of Great Britain, America, And Other Countries. Their Breeding, Training, and Management in Health and Disease", by John Henry Walsh (Stonehenge). Also available from Amazon: The Dogs Of Great Britain, America And Other Countries.
Terriers are entered to vermin with great facility, and require very little breaking, unless they are intended to be used with ferrets. Then they must be broken to let these animals alone, as they are apt to make their appearance occasionally in passing from one hole to another. It is only necessary to let the ferret and the terrier be together in a yard or stable for a few times, cautioning the latter not to touch the former, and the young dog soon learns to distinguish his friends from his foes. Some terriers are not hardy enough to brave the bites which they are liable to in ratting, etc., and, indeed, the true terrier without any cross of the bull-dog is a great coward, so that he is quite useless for the purpose. In such a case, he must be encouraged by letting him kill young rats at first, and as he gains confidence, he will perhaps also increase in courage, If, however, the terrier is well bred, he will rarely require anything more than practice.