Interdigital webbing is found among many vertebrates in the early stages of development. However, many of these organisms remove this webbing through tissue-specific apoptosis, removing the interdigital membrane back towards the digits. Some mammals retain this webbing, such as bats. This is feasible through inhibition of apoptosis, specifically through the BMP inhibitor Gremlin.1 BMP signaling typically results in apoptosis in expressed tissues, but through Gre inhibition, these tissues remain and can be repurposed-in the case of bats-for flight.
(1) Weatherbee, S. D., Behringer, R. R., Rasweiler, J. J., & Niswander, L. A. (2006). Interdigital webbing retention in bat wings illustrates genetic changes underlying amniote limb diversification. Proceedings of the National Academy of Sciences, 103(41), 15103–15107. doi: 10.1073/pnas.0604934103