To Darwin, realizing that the perturbation of organismal development can lead to the expression of ancestral characters, hidden like invisible ink, overwhelmingly demonstrated to him the existence of heredity. More generally, the ubiquitous occurrence of ancestral reversions, known as atavisms, solidified the principle of common descent, and the evolutionary tree of life. Yet Darwin went beyond the acknowledgment of their existence and wondered how organisms can retain a latent capacity, the “power of calling back to life long-lost characters”, to produce ancestral traits, which can be reactivated by changed conditions. This remarkable dual capacity of retaining and re-expressing lost ancestral traits has been almost completely underappreciated by the mainstream evolutionary community. Here, I propose that the underlying developmental mechanism of Darwin’s concept of Invisible Ink, although relegated to obscurity, is critical to the evolutionary process. Inspired by findings from my own work, ranging from ants to cartilaginous fish, and that of others, we (Ehab Abouheif & myself) will attempt to construct a synthesis, which resurrects this concept’s importance. Rather than hopeful or hopeless monsters, or solely evidence for common descent, this developmental capacity is important and represents ancestral developmental potentials (ADP). ADP can be retained (for potentially millions of years), in a hidden form, by molecular processes such that when induced, it can serve as raw materials for natural selection. Furthermore, this ancient variation is not historically blind to selection; rather it is potentially preadaptive, and can be induced by recurrent environmental and genetic conditions, facilitating parallel evolution and novelty.