. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . Haeckel summedup in the phrase, ontogeny isa recapitulation of phylogeny,must not be too heavily leanedon as a support for any specula-tions as to the phyletic affinities of any species or group ofspecies of organisms. Embryology is an ancient manuscriptwith many of the sheets lost, others displaced, and withspurious passages interpolated by a later hand. While a young robin when it hatches from the egg or ayoung kitten at birth resembles its parents, a y
. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . Haeckel summedup in the phrase, ontogeny isa recapitulation of phylogeny,must not be too heavily leanedon as a support for any specula-tions as to the phyletic affinities of any species or group ofspecies of organisms. Embryology is an ancient manuscriptwith many of the sheets lost, others displaced, and withspurious passages interpolated by a later hand. While a young robin when it hatches from the egg or ayoung kitten at birth resembles its parents, a young starfishor a young crab or a young butterfly when hatched does notat all resemble its parents. And while the young robin afterhatching becomes a fully grown robin simply by growing largerand undergoing comparatively slight developmental changes,the young starfish or young butterfly not only grows larger,but undergoes some very striking developmental changes; thebody changes very much in appearance. Marked changes inthe body of an animal during postembryonic or larval de-velopment constitute what is called metamorphic development,. FIG. 137.—Metamorphosis of a bar-nacle, Lepas: a, Larva; 6, adult. GENERATION, SEX AND ONTOGENY 235 or the animal is said to undergo or to show metamorphosis inits development. This metamorphosis is familiar to all in insects; to zoologists,it is familiar among numerous other kinds of animals. Fig. 138
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