Today I discovered the source for a few images I've around the internet on the development of particular arms and armour, and I thought you might all like to have a look at it:

Development of Arms and Armour, by Bashford Dean.

Dean was a zoologist by trade, but had become a self taught expert in medieval arms and armour and was the founder of the Metropolitan Museum of Art's Department of Arms and Armour. As curator, he put together a few drawings showing the evolution of weapons and armour as he saw it.

I should stress that some parts of Dean's conclusions are badly out of date. Chapel de fers, jezants ballistae, composite bows and composite prod crossbows all developed before Dean thought they did and, in some cases, had ceased being in use before he thought their use had ended. I'm also not sure how he misinterpreted what a poleaxe was so badly.

However, that said, the drawings are still extremely valuable as a visual aid and, when all is said and done, good enough for fantasy.