CREATION BY EVOLUTION
vertebrates, modifications due to the presence of much yolk, and extraordinary modifications of parasitic larvae. Such modifications indicate that evolutionary changes may occur in embryonic as well as in adult stages — in fact, that they may affect any part of the life history. But this does not disprove the thesis that in general “ontogeny recapitulates phylogeny.” The fundamental resemblances or homologies between embryos, larvae, and adults which have been cited above are just as genuine homologies as those between adult structures, and the only natural explanation that has ever been found for such homologies is inheritance from common ancestors.
The origin of the individual (ontogeny) is not only a key to the conditions and causes of the origin of the race (phylogeny) but is also an actual evolution — that is, an origin of new forms by transformation from old ones. In the long series of stages which an egg passes through in its transformation into a mature animal one sees not only an actual evolution — the evolution of an individual — but also, more or less obscurely, a repetition of the stages of the long-past evolution of the ancestors of the species.
Man no less than other mammals develops from a fertilized egg, which passes through cleavage, blastula, and gastrula stages. The human embryo has gill slits and aortic arches, which undergo exactly the same transformations that take place in other mammals. Man's heart is at first like that of a fish, consisting of one auricle and one ventricle. His backbone begins as a notochord, is next a segmented cartilaginous rod, then each segment or vertebra consists of five separate bones, and finally each fuses into a single bone. He has in the course of his development three different pairs of kidneys, first a pronephros (or fore-kidney), like that of the lower fishes, then a mesonephros (or mid-kidney), like
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