Cross-Species genetics was a theory shared by Dr. Curt Connors and Richard Parker.
Dr. Curt Connors was a man with only one arm and a dream, that humanity could live in a world without weakness. Him and Richard Parker who shared this dream were working on a medicine within Oscorp[company made by highly smart Norman Osborn] that could give everyone perfect physical perfection and no weakness (for example, be able to re-grow limbs) and they were both near finishing this project until Richard, unfortunately, died taking all his research with him; delaying this project by another 12 years. Connors planned to distribute it and also cure himself, by re-growing a new arm. He believed he could do this not with stem cells, but with the use of cross-species genetics using the abilities of a Lizard's biology (being able to re-grow limbs) and apply this ability into a human body.