Paleoclimate data inferred indirectly via the use of transfer functions. The underlying idea is that organisms exhibit a high degree of differentiation according to their physical environment, and that physical variables can be estimated from biotic distributions once the degree of relationship has been objectively established. For example, some present plankton species live in cold waters and others prefer warmer waters. If we make the additional assumption that fossil assemblages of these species (or their related ancestors) exhibited similar temperature tendencies, then we can infer, within limits, the temperature of the water in which they existed.