The animals on the cover of Learning Rails are tarpans (Equus ferus ferus). The tarpan was a wild horse that lived in Europe and Asia and died out in the 19th century. Smaller and stockier than a modern domestic horse, it was mouse-gray in color with a dark mane and a black stripe down its back. The breed was known to be intelligent, curious, and independent.

The ancient tarpan ranged from southern France and Spain to central Russia. Its decline was caused by the growth of the European human population in the 17th and 18th centuries, which encroached on the tarpan's natural habitat. Tarpans were also hunted for their meat. The last wild tarpan died in the Ukraine in 1879, and the last pure tarpan died in a Russian zoo eight years later, at which point the species officially became extinct.

However, you can still see a tarpan today, thanks to two German zoologists who succeeded in genetically recreating the breed in the 1930s. Heinz and Lutz Heck began a breeding program while working at a Munich zoo, believing that genes still present in the gene pool of an overall species could be used to recreate extinct breeds. They combined the genes of living horses who showed similar characteristics to the ancient tarpan, and bred the first modern tarpan at the zoo in 1933. This new form of tarpan, known as the Heck horse, is a phenotypic copy of the original wild breed, meaning that it resembles the ancient tarpan but is not exactly the same genetically. Today, there are about 50 tarpans in North America, all of which trace back to the original project in Munich. Most of them are owned by private breeders who are trying to increase the tarpan population. There are not many more than 100 tarpans in the world.

The cover image is from Richard Lydekker's Royal Natural History. The cover font is Adobe ITC Garamond. The text font is Linotype Birka; the heading font is Adobe Myriad Condensed; and the code font is LucasFont's TheSansMonoCondensed.