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111 Special homology of vertebrate limbs 113 Fore-limbs—Comparison of fore-limbs of mammals, birds, reptiles, and fishes, part for part—Gradual changes in collar-bone and coracoid—In position of elbow—In bones of forearm—In position of wrist—In the tread—The term manus—Number of toes—Modifications for flight in various animals—For swimming in whales and fishes 113 Hind-limbs—Comparison of hind-limbs of several mammals—Position of knee—Of heel—Plantigrade and digitigrade—Degrees of the latter—Number of toes—General law in regard to number of similar parts—Order of toe-dropping in artiodactyles—In perissodactyles 121 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 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 CHAPTER VIII. PROOFS FROM GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. Geographical faunas and floras—Conditions which limit 183 Temperature-regions—Illustrated by plants—In latitude and in elevation—Same in animal species 184 More perfect definition of regions—Range