Charles Darwin’s sketch of how species are related is a tad messy but I love it. So I simplified the lines and nudged the “I think” notation to a different location. Because people asked, I put my image on Redbubble, a company that prints images onto a variety of items (see sample gallery below). White is displayed but they can alter everything. I especially like the sticker. For dark t-shirts and such (bottom row in gallery below) use this link. If you don’t see what you want or you’d prefer the image be in a different location, just let me know.
I also have a Redbubble page for the design without the “I think” text. E.g., travel mug below.