Embryonic Recapitulation

By considering the embryological structure of man - the homologies which he presents with the lower animals - the rudiments which he retains - and the reversions to which he is liable, we can partly recall in imagination the former condition of our early progenitors; and we can approximately place them in their proper position in the zoological series. We thus learnt that man is descended from a hairy quadruped, furnished with a tail and pointed ears, probably arboreal in its habit, and an in...

 If Birds Studied Man

Let us only imagine that birds had studied their own development and that it was they in turn who investigated the structure of the adult mammal and of man. Wouldn't their physiological textbooks teach the following? 'Those four and two-legged animals bear many resemblances to embryos, for their cranial bones are separated, and they have no beak, just as we do in the first live or six days of incubation; their extremities are all very much alike, as ours are for about the same period; there i...

 Ontogeny Recapitulates Phylogeny

Now, we’re not absolutely sure why some species retain much of their evolutionary history during development. The “adding new stuff onto old” principle is just a hypothesis—an explanation for the facts of embryology. It’s hard to prove that it was easier for a developmental program to evolve one way rather than another. But the facts of embryology remain, and make sense only in light of evolution. All vertebrates begin development looking like embryonic fish because we all descended...