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Early embryologists noticed similarities between

Early embryologists noticed similarities between - von Baer...

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Early embryologists noticed similarities between ontogeny (the development of an organism) and phylogeny (ancestor descendant relationships in a group). The common phrase "ontogeny recapitulates phylogeny" was put forward by Haeckel as his biogenetic law (see fig. 21.3, pg. 588). Haeckel held that descendants, during their ontogeny, passed through stages that resembled the adults of their ancestors. Before this, Cuvier (1812) held that there were four major classes of organisms: vertebrates, mollusks, articulates and radiates. Cuvier noticed that there was nothing in the ontogeny of a vertebrate that resembled the adult stages of, say, a mollusk. This is because evolution is a bush or a tree not a "ladder" of the great chain of beings. This "branch-like" pattern to phylogeny was apparent to Haeckel, but he still claimed there was "recapitulation".
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Unformatted text preview: von Baer made observations about ontogeny and phylogeny that seem obvious to us today, but they are important in development and evolution as they run counter to recapitulation : 1 ) more general characters appear early in development, 2 ) less general forms develop from the more general forms, 3 ) embryos do not pass through other forms they diverge from them, 4 ) embryos of higher forms only resemble embryos of other forms (human, calf, chick and fish look similar at embryo stage but diverge quickly). See section 17.8.2, and fig. 17.11, pgs. 478-479. Putting these two views together, we see that there can be a sort of recapitulation within a lineage (i.e., within an evolutionary sequence of ontogenies) but there are many examples that refute the notion that phylogeny is reviewed during ontogeny....
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