I have some info on the evolution of the Highland dirk:
In Levine's Guide to Knives and their Values (1985),
ISBN 0-910676-94-1 , page 438 is the reproduction of a graphic of the "Development of Daggers from XIII-XVIII century".
Without permission to reproduce I'll translate it to words:
We start in 1300 with two main types of daggers (utility and hunting knives are not in this picture)
Type A seems double edged with two lobes at the hilt and a similar pommel (1300) the line splits in three around 1400:
A1:The line of the ballock or kidney daggers that evolve up to 1550.
A2: the Highland dirks who evolve similar to the kidney dagger (until 1500)but the lobes get less distinct and get flatter. Single edges get more common and the "typical" highland dirk appeares around 1600 to the 18th century when the "Victorian Dress Dirk"
appeared.
A3 is a shape in between A1 and A2 but not detailed.
Type B seems to have a wider blade and a ring at the pommel (1300)and has four main lines evolving:
B1:Roundel daggers become Landsknechtdaggers(1550-1600).
B2: Baselard daggers (up to 1500's) and swiss daggers (up to 1600's)
B3: Quillion daggers evolve (around 1550's) in Poingnards(B3.1)
stylets(B3.2) ,plug bayonets(B3.3) and left hand daggers(B3.4).
B4: Ring daggers become eared daggers from 1400-1600's.
The Fall 1999 issue of Swordforum is great about the Arms of the Scots and links to Michael McRae and others:
http://www.swordforum.com/fall99/main.html
I have seen and handled Highland dirks in many shapes : light and fast,well balanced fighters up to heavy blades, some leaning towards saexes . Many dirks were made from broken swords, what often dictated final shape.
To me the Victorian dirk is more jewelry and the stylised thistle shapes are uncomfortable in the hand.
Jan