the organization of the egg and the localization of organ- forming substances can vary greatly in closely related organisms (pp. 136, 151, 162), and embryonic development can take short cuts (p. 42) without evolutionary significance, it may be questioned whether such differences in the egg originated the evolution of these groups instead of accompanying or following it, and still more whether the clear-cut differences between the eggs of such groups today is evidence for the suddenness of their appearance in evolution.


Conclusion

It is clear, then, that evolutionary novelties do arise in the early stages of development and may restrict their effects to those stages. It is equally clear, as Morgan ( 1924) pointed out, that the internal factors controlling the production of these novelties can be and are transmitted like any other internal factors, i.e. are genes. We therefore reach the conclusion that these youthful characters are prone to the effects of heterochrony by acceleration or retardation of the rates at which the internal factors act, and might therefore be expected to appear in the adult as well as in the youthful stages. This possibility will be considered in a subsequent chapter.

A word is, perhaps, necessary to justify the retention here of the term 'caenogenetic'. It is meant solely to designate the origin of an evolutionary novelty in early stages of ontogeny. Shorn of the implications which Haeckel attributed to it, it might seem as though the term were better dropped. But the terminology is already complicated and confused, and since the obvious alternative term 'neogenetic' is already used in another sense (see p. 37), it appears to be simplest to retain 'caenogenetic' and restrict its meaning as above.

-51-

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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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