XIII
ACCELERATION

WE have now to consider the last of the possible results that heterochrony may produce. By acceleration of the rate of action of the internal factors which control its formation, a character which appeared in the late stages of ontogeny of an ancestor may appear early in the development of a descendant. Swinnerton's term 'deuterogenesis' applies to those cases in which the character in the adult descendant is farther developed than in the adult ancestor, owing to its acceleration. In other cases the accelerated character may no longer appear in the adult stage of the descendant. The embryonic or larval features of the descendant may then resemble the adult stage of the ancestor. The conditions required for Haeckel's theory of recapitulation will then be fulfilled, but again with the remark that accelerated repetition of a character in the ontogeny of the descendant does not constitute a pressing back of a complete ancestral adult stage into an earlier period of ontogeny. The following examples of acceleration may now be considered.

It is not always easy to be certain that a case which appears to show acceleration is not really one of hypermorphosis. This is especially true when the character in question is associated with a shell formed by successive increments. Acceleration and hypermorphosis are, of course, not mutually exclusive, for in the evolution of Gyphaea, as we have seen (p. 101), the angle of the coil of the older part of the shell of the ancestor is reproduced in the younger part of the shell of the descendant.

A case in many ways analogous is provided by the Foraminifera. The inner whorls of the coiled shell of Cycloclypeus repeat the formation of chambers characteristic of the probably ancestral form Heterostegina, as Tan Sin Hok has shown. But the number of these heterostegine chambers diminishes in successive species of Cycloclypeus from 38 in the Oligocene to 3 in the Pleistocene.

It had been thought by Schindewolf and others that the series of Foraminifera represented by Nubecularia, Ophthalmidium, Spirophthalmidium, and Spiroculina presented an example of neoteny,

-104-

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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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