I learned allot by going through these very fast and visual lectures. I am sharing the link with you so you can explore too. http://gds.parkland.edu/gds/!lectures/ I have put two below to because they both show Design History in regards to logos. The first is a "what if" with Coppertone and the second is what really did happen with the Prudential logo.
The Prudential Logo has gone through many design changes that reflected the current trends. Check it out here to see and because of our study, we can easily see what time period each is.