What specific feature of the Waterside Terrier lent significant spirit to the emerging Yorkie line?
Proficiency at catching water rats
The foundation stock for the Yorkshire Terrier was a composite mix of several smaller terriers used in the industrial north. Among these key contributing ancestors was the Waterside Terrier. This specific terrier type was known for several traits beneficial to the development of the Yorkie, including a rough coat and a suitable size for close work. Most importantly, however, the Waterside Terrier demonstrated a high degree of competence and skill in catching water rats. This expertise contributed the necessary terrier spirit, bravery, and effectiveness in vermin control that breeders needed to preserve in the developing lineage, ensuring the resulting dog was tenacious despite its small frame.

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Yorkshire Terrier - Full History - YouTube