XV
REPETITION

WE have now come nearly to the end of our review of the relations which ontogeny and phylogeny bear to one another, and we have seen that the characters which appeared in the ontogeny of the ancestor tend to reappear in the ontogeny of that descendant, either at a corresponding stage of development, or earlier, or later.

That the reappearance of characters mostly takes place at corresponding stages was clear to Darwin when he wrote 'at whatever age a variation first appears in the parent, it tends to reappear at a corresponding age in the offspring.' The possibility of change in the time-relations was also clear to him, for he continued: 'variations, which, for all we can see might have first appeared either earlier or later in life, likewise tend to reappear at a corresponding age in the offspring and parent. I am far from meaning that this is invariably the case.'

We have seen that that which is repeated in the ontogeny of the descendant may represent the embryonic or larval just as well as the adult characters of the ancestor, and that the retarded repetition of youthful ancestral stages is of particular importance because of the part which it plays in paedomorphosis. The appearance of new characters in the early stages of development is caenogenesis, and these characters which loom so largely in neoteny and deviation are flies in the Haeckelian ointment of recapitulation, for this theory was bound to treat them as exceptions to his rule of evolution. It is because these early-developed characters are not exceptions in phylogeny, together with the fact that phylogeny is the result of ontogeny instead of being its cause, that we reject Haeckel's theory of recapitulation.

We have also seen that the accelerated appearance of a character in the ontogeny of a descendant cannot be construed as the pressing back of a complete adult ancestral stage into earlier stages of development, especially as, accompanying the acceleration of some characters, there may be neoteny of others (example: graptolites). Evolution does not take place by the addition or deletion of 'stages' to or from successive ontogenies.

-125-

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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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