The New Book of Dog (1907), gives you an idea of how early breeders used different breeds in order to achieve the look of the dog that we today call the Yorkshire Terrier.
The new book of the dog; a comprehensive natural history of British dogs and their foreign relatives, with chapters on law, breeding, kennel management, and veterinary treatment
In the Yorkshire Terrier Chapter, it talks about using crosses of dogs like the Clydesdale, Skye, Maltese and Dandi and when the litter is produced, making selections and rejections of the pups based on characteristics that they wanted or didn't want ...
"Evidence of origin is often to be found more distinclty in puppies than in mature dog, and it is to be noted that the puppies of both the dandie and yorkshire are born with decided black and tan coloring.* Selection and rejection must have been important factors in the production-selection of offspring which came nearest to the preconceived model, rejection of all that had the long body and short legs of the skye, the white colouring of the maltese, the drooping ears of the dandie and the wiry coat of the black and tan"