In standard textbook definitions vestiges are no longer functional.

In biological terms, they reveal former evolutionary and developmental conditions, and in this way they offer valuable keys to unlock the past.

Like actual footprints, vestiges offer glimpses of what was once there but is no longer present. Vestiges (from the Latin vestigium, for footprint) reveal traces of prior states. Life is a dynamic process, and organismal structures disclose the history of the two principal levels of biological change: evolutionary and developmental. These holdovers are useful in differentiating and relating concepts of phylogeny and ontogeny as well as revealing benefits of historical reasoning in understanding patterns and processes of organismal change. This paper presents examples of developmental remnants, which often involve circulatory and reproductive alterations, and discusses numerous other retained or reappearing historical holdovers in ways that reveal the unfolding dynamic interaction between genotype and phenotype. To avoid confusion, the term vestige should be reserved for true evolutionary holdovers. In addition to remnants of evolutionary change, traces of developmental change likewise exist, and it is important to distinguish embryonic remains from true evolutionary vestiges because people confuse evolutionary and developmental changes. Such features include not only anatomical structures but also physiological processes, biochemical reactions, and even behaviors. Vestigial features of humans and other organisms are well known and have long been used as key evidence for evolution.

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