IV
PHYLOGENY

PHYLOGENY is the scale of beings rehabilitated as a result of the theory of evolution into a row (or rather a number of rows) of adult forms which are related to one another, not from adult to adult, but from the fertilized egg which gives rise to one adult to the fertilized egg which produces the next. The adult forms in phylogeny are therefore disconnected from one another and incapable of influencing their successors, since this would require that the effects of use and disuse could become converted into internal factors and so affect the ontogeny and production of the adult of the next generation.


Phylogenetic transformations

It was pointed out by Garstang ( 1922), Swinnerton, and Kryžanowsky that the restriction of the term phylogeny to the adults of the series is arbitrary, and that phylogeny should be regarded as the succession of complete ontogenies. This is true, and it must be noted that the upholders of the theory of recapitulation had adults in mind when they thought of phylogeny. But the cases in which the ontogenies of the animals in a phylogenetic series are known (ammonites, gastropods) are few. It is, therefore, more convenient to restrict phylogeny provisionally to mean the succession of adults. If all animals had been preserved as fossils, and if all these fossil remains had been discovered, we should be able to trace a continuous series of adult ancestral forms which would represent the phylogeny of the race which we were studying. Unfortunately this is not the case for the fossil record is still imperfect, and we only have a number of more or less isolated forms to indicate the track which phylogeny has taken. Nevertheless, the study of the phylogenies of a number of animals such as the horse, elephant, and camel have yielded very interesting series showing the progressive modifications which these races have undergone during evolution. Now, on comparing the various members of a phylogenetic series, it appears that the differences between the structures of the adults are largely differences of proportion, and of number, of the

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Embryos and Ancestors
Table of contents

Table of contents

  • Title Page iii
  • Preface v
  • Contents vii
  • List of Illustrations xi
  • I- Stages of Development And Stages of Evolution 1
  • II- Ontogeny 14
  • III- The Speeds of the Processes Of Development 22
  • IV- Phylogeny 29
  • V- Heterochrony 34
  • VI- Caenogenesis 40
  • Conclusion 51
  • VII- Deviation 52
  • Conclusion 61
  • VIII- Neoteny 63
  • Conclusion 90
  • IX- Vestigial Structures Due To Reduction 92
  • X- Adult Variation 97
  • XI- Vestigial Structures Due To Retardation 99
  • XII- Hypermorphosis 100
  • XIII- Acceleration 104
  • Conclusion 110
  • XIV- Paedomorphosis And Gerontomorphosis 111
  • XV- Repetition 125
  • XVI- Embryology and Taxonomy 134
  • XVII- Embryology and Homology 146
  • XVIII- The Germ Layers 154
  • Conclusions 168
  • XIX- Conclusions 170
  • Bibliography 175
  • Index 191
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