The idea of internal selection, originally proposed by Lancelot Whyte, not only helps us to understand the causal status of developmental constraints, but can also show us how these organismic or internal factors, whose relevancy Evolutionary Developmental Biology highlights today, can be considered from a variational or selectional perspective that is compatible with, but not reducible to Natural Selection Theory. Thus, since it is considered as a concept that is autonomous and different from natural selection, the concept of internal selection can enable us to understand how the phenomenon that Wallace Arthur calls developmental bias constitutes an effective and positive cause of cumulative and irreversible evolutionary changes.