In the cases cited above, in humans the stages are actually not progressive steps to an adult kidney, but are three different fully-functional excretory organs, and only the last stage is retained into adulthood (Davis et al. 1989, 129). Another major problem with this theory is that some embryos of very closely allied species follow very different embryonic paths, not only of kidney development, but of other organ development as well. This kidney recapitulation claim was in the past seen as one of, if not the best, example of recapitulation, but for good reasons is rarely cited in evolutionary textbooks today (Shute 1971, 40–41). Also, many kidney traits are not consistent with mammal evolution.
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