. The study of animal life . s is in great part due, though subsequent studiesin development have added welcome corroboration to manyof the comparisons which formerly were based solely on theresults of anatomy. Thus an organ derived from the outerembrj^onic layer cannot be homologous with one derivedfrom the innermost stratum of embrjonic cells. Homo-logous organs in one animal are well illustrated by the CHAP. XI The Elements of Structure 111 nineteen pairs of appendages borne by a crayfish or differ greatly in form and in function ; many of themare not analogous with their neig
. The study of animal life . s is in great part due, though subsequent studiesin development have added welcome corroboration to manyof the comparisons which formerly were based solely on theresults of anatomy. Thus an organ derived from the outerembrj^onic layer cannot be homologous with one derivedfrom the innermost stratum of embrjonic cells. Homo-logous organs in one animal are well illustrated by the CHAP. XI The Elements of Structure 111 nineteen pairs of appendages borne by a crayfish or differ greatly in form and in function ; many of themare not analogous with their neighbours, one feels andanother bites, one seizes and another swims, but they areall homologous. So are the different forms of fore-limb,the pectoral fin of a fish, the fore-leg of a frog or lizard, thewing of a bird, the flipper of a whale, the fore-leg of a tiger,the arm of man. But the wing of an insect is merelyanalogous not homologous with that of a bird, while thewings of bats and birds are both analogous and Fig. 33.—Bones of the wing in pigeon (A), bat (B), extinct pterodactyl (C).(From Chamberss Encyctop.) Change of Function.—Organs are not mechanisms rigidlyadapted for only one purpose. In most cases they have amain function and several subsidiary functions, and changesmay take place in organs by the occasional predominanceof a subsidiary function over the original primary the swim- or air-bladder which grows out dorsallyfrom the food-canal of most fishes, seems usually to be ahydrostatic organ; in a few cases it helps slightly inrespiration, but in the double-breathing mud-fishes orDipnoi it has become a genuine lung. An unimportant(allantoic) bladder at the hind end of the gut in frogs, isrepresented in the embryos of reptiles and birds by a veryimportant respiratory (and sometimes yolk-absorbing) birth- N 178 TJie Study of Ajtimal Life part hi robe, and in almost all mammals by part of the placentawhich unites mother and unborn offspring. Su
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