Embryological criterion

The embryological criteria is derived from Karl Ernst von Baer's (1792 - 1876) first embryological law:

"The general features of a large group of animals appear earlier in the embryo than the special features”.

Cartilage, for example, is found in all fish - in cartilaginous fish such as sharks as well as in bony fish like these salmon pictured opposite. Cartilage is a general character; bone is a special character, being found only in bony fish. Von Baer's law correctly predicts that, in bony fish, cartilage will appear earlier in individual development, and will transform into bone.

The characters von Baer called 'general' are in evolutionary terms ancestral; and his 'special' characters are evolutionarily derived: the successive transformations from general to special forms of a character are evolutionary changes between ancestral and derived character states.

By the embryological criterion, cartilage is inferred to be an ancestral state, bone derived: the bone in bony fish evolved from a cartilaginous ancestry. In general, if we have a group of species and a list of homologous characters, then (if they are the kind of characters that undergo development) the relative ancestral and derived character states can be inferred from their order in development.

The embryological criterion only works when von Baer's law is correct and while it is widely accepted to have some truth, it is also known to have exceptions.