Increase of the ratio (R) of the number of vomeronasal (or odorant) receptors for airborne chemicals to the number of receptors for water-soluble molecules in the vertebrate transition from water to land. For vomeronasal receptors, R is the number of intact V1R genes divided by the number of intact V2R genes in a species. For odorant receptors, R is the number of intact class II OR genes divided by the number of intact class I OR genes in a species. The numbers of odorant receptor genes in various vertebrates are from Olender et al. (2004), Niimura and Nei (2005), and Quignon et al. (2005). R is not shown if it is infinity.