Evolution and the theory of natural selection are the principle unifying concepts in biology. This is as true now in the age of molecular biology as it was a century ago. The unification stems from the fact that details of embryology, morphology, molecular biology, physiology, paleontology, animal distribution, etc. only become understandable when considered in the context of evolution and natural selection. Certainly evolution had been considered prior to the publication of Charles Robert Darwin's (1809-1882) the The Origin of Species in 1859. However, a mere consideration was no longer appropriate for biological scientists. Darwin's book was an extensive catalog containing a plethora of evidences that support the reality of evolution. His theory of natural selection provided the mechanism for evolution. The 1859 book was the beginning of modern biology.

Evolutionary theory explains the bewildering array of similarities in related biological groups. For instance, frogs, mice, and humans...