_Genesis of the horse_--Changes in foot-structure--Same true of other parts of skeleton--Only natural explanation is derivation--Nature compared with man in mode of working-- Angels--Griffins--Centaurs--Muscular and nervous systems --Visceral organs 126 CHAPTER VI. HOMOLOGIES OF THE ARTICULATE SKELETON. _Illustrations from this type_--Plan of structure entirely different--General plan of structure explained and modifications shown--Shrimp--Modification of segments and of appendages for various purposes: swimming, walking, eating, sense--Illustrated by other crustaceans--By myriapods-- By marine worms--Crabs--Embryonic development of crabs-- insects--Modification of segments and appendages--Mouth parts of insects 132 _Illustration of the law of differentiation_--Cells--Segments --Individuals--Homologies in other departments of animals, but these are less familiar--Between primary groups, homology untraceable in adult forms--But these also probably connected by common origin--Different views as to origin of vertebrates 144 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