Body plans are extremely stable. Early stages of individual development are remarkably resistant to evolutionary change, either because they are resistant to mutations or have extremely efficient repair mechanisms. Only later appearing, superficial features of the animal, such as coloration, are subject to adaptive change. Evolution thus does not change the body plan and other early-appearing features of the embryo. Phyletic body plans have not changed in more than 500 million years, with the possible exception of some reformulation of sea urchins' body plans.
Development proceeds in characteristic time sequences and directions.
This is the top-down direction of development. This is the upside-down developmental phyletic tree. The top-down direction of both individual and phyletic development represents a realistic link between individual and large scale development, as noted by K. S. Thomson. The shape of the developmental life span is curvilinear. The entire life span of individual animals is a manifestation of development.