CHAPTER VII.

PROOFS FROM EMBRYOLOGY, OR COMPARISON IN THE ONTOGENIC SERIES.

_Resemblance of the three series_--Frog, in Ontogeny passes through main stages of Taxonomy and Phylogeny--Resemblance only general--Many steps dropped out in Ontogeny 148

(1) _Ontogeny of tailless amphibians_--The frog: fish stage, perennibranch stage, caducibranch stage, aneural stage--Same stages in Phylogeny 150

(2) _Aortic arches_--Those of lizard described--Origin from gill-arches of fish--Change from one to the other in Ontogeny of a frog--Same changes in Phylogeny of lizard--Embryonic condition of mammalian heart and vessels--Gradual change and final condition in birds--In mammals--Gradual decrease in number of aortic arches as we go up the vertebrate scale-- Cogency of the argument from aortic arches 151

(3) _Vertebrate brain_--Fish brain--Brain of reptiles, birds, mammals, man compared--Human brain passes through similar stages--Changes in complexity of structure in Taxonomy-- Same changes in Ontogeny of mammals--Same in Phylogeny of reptiles, birds, mammals 162

_Cephalization_--Explanation of, in body, in mind 171

(4) _Fish-tails_--Homocercal and heterocercal--Vertebrated and non-vertebrated--Order of change in Ontogeny--Same in Phylogeny--Similar changes in birds’ tails in Ontogeny and Phylogeny--In other tailless animals--Examples from articulates, insects, crustaceans, etc. 172

_Illustration of the differentiation of the whole animal kingdom_ --Development of eggs of all kinds of animals--This a type of changes in Phylogeny--Why Ontogeny repeats Phylogeny-- Law of acceleration 176

_Proofs from rudimentary and useless organs_--Examples from whale: Teeth--Limbs--Hair--Olfactive organs--Examples from man: muscles, cæcal appendage--Significance of useless organs 179