Much of the 2003 edition of this book is posted online in Google Books. While it isn't the same as having a copy of one's own, it does make it possible to do some research. The concluding chapter, Breeding and Selection by Poultry Fanciers, is written by W.C. Carefoot. Dr. Carefoot was president of the Poultry Club of Great Britain. [Unfortunate that they couldn't have found an American expert.] He credits fanciers with playing a "significant role in providing the raw material on which the poulry industry is based."
"Breeders have out of sheer pleasure conserved the large gene pool of mutations which have spontaeously occurred over the centuries...The production by a skillful breeder of high quality birds of a particular variety will attract sufficient interest to ensure preservation and improvement....Indeed, where a fancier has taken up a breed that is in serious decline and has bred them aggressively, hatched as many chicks as possible, and rigorously culled weaklings and birds with serious defects, it has been amazing how the followers of the particular variety have multiplied."
The Buckeye and the Java are two examples of breeds that have gained followers as small flock keepers learned about them. The Auburn Java is a variety that was lost and is being recovered. Read my report on this remarkable re-emergence in this month's Backyard Poultry magazine. You'll have to get your own copy -- the article isn't posted online.