Final answer:
Charles Darwin's theories were expanded upon by later scientists, particularly with the discovery of genetics, which provided the mechanisms for inheritance and variability. Alfred Russel Wallace independently conceived a similar theory of evolution by natural selection, prompting Darwin to publish his comprehensive work, On the Origin of Species.
Step-by-step explanation:
The individuals who filled in the blanks of Charles Darwin's theories were primarily other scientists and researchers who came after him. Darwin, along with Alfred Russel Wallace, proposed the theory of evolution by natural selection. Both of these naturalists made significant contributions to the understanding of evolution based on their respective journeys and observations. For example, the variations in the beak shapes of finches that Darwin observed in the Galápagos Islands provided key evidence of adaptation and the process of natural selection. It was after receiving Wallace's paper on evolution that Darwin was compelled to finalize and publish his seminal work, On the Origin of Species. This book solidified the theory of evolution by providing compelling arguments and evidence.
Eventually, the field of genetics, particularly through the work of Gregor Mendel and later scientists who discovered DNA, provided the mechanisms by which traits are inherited and variability is produced, addressing some of the remaining questions from Darwin's time. These scientific advancements allowed for a deeper understanding of the process of evolution, thereby expanding upon Darwin's foundational work.