Ernst Haeckel's sketches led to the production of a thousand engravings over the course of his career. A hundred of them were selected to be in the book Kunstformen der Natur (Art Forms of Nature), where Heackel's sketches were engraved on a lithographer by Adolf Giltsch, and then printed. The images below are from this book.
It was absolutely impossible to chose which prints to present below, and which ones to leave out. They were all incredibly beautiful, and I felt like putting up all hundred prints. But in the end, I chose the ones I liked best, so that your computer doesn't collapse with the weight of downloading 200 MB worth of images.
All these images were downloaded via the Biodiversity Heritage Library, and they have the entire book online to read, and download for free. Seriously, you should check out that website, because their collection is amazing. I could stay lost there forever, turning the digital pages of some of the most beautiful illustrations of nature ever made.
(Click on an image to view a larger version.)