The pictures, above, show what happened to the Bull Terrier between 1930, 1950 and 1980, as breeders selected for clinorhynchy, or dorsoventral nose bend -- a downward nose droop.
Bull terriers have purposefully selected for this genetic mutation, creating a dramatic case of evolution visible in the short time we have had modern photography. As Fondon and Garner note,
"The dramatic changes that have occurred in domestic dog breeds in response to breeders’ selection toward breed standards over the last 150 years demonstrate the potential of the mammalian genome to effect rapid morphological change in response to strong selection, even with small, closed gene pools."
To see more dramatic pictures of dog skull evolution, see >> Molecular origins of rapid and continuous morphological evolution by John W. Fondon III and Harold R. Garner
To put these changes to the dog within context of working terriers, see >> Rosettes to Ruin.