I'm not sure why it's useless. If you're going to account for value provided by external sources then account for all sources, not just the ones that support your argument.
Your 100 year cut-off point is entirely arbitrary and chosen only because it supports your arguments.
[1] http://en.wikipedia.org/wiki/History_of_mathematics
[2] http://en.wikipedia.org/wiki/Origin_of_language
[3] http://en.wikipedia.org/wiki/History_of_writing
[4] http://en.wikipedia.org/wiki/Printing_press
[5] http://en.wikipedia.org/wiki/History_of_scientific_method