Embryological resemblances of all kinds can be accounted for, as we shall hereafter see, by the progenitors of our existing species having varied after early youth, and having transmitted their newly-acquired characters to their offspring, at a corresponding age.

The embryo is thus left almost unaffected, and serves as a record of the past condition of the species.

Hence it is that existing species during the early stages of their development so often resemble ancient and extinct forms belonging to the same class.