XI
VESTIGIAL STRUCTURES DUE TO RETARDATION

WHEN, as in neoteny, characters which had been larval or embryonic in the ancestor become adult in the descendant, the original adult characters of the ancestor tend, as it were, to be pushed off from the end of ontogeny. They arise too late to be fully formed by the time maturity is reached in those animals which have a definite adult form, and, in consequence, such characters become reduced and vestigial. Retardation of structures to vestiges is therefore the other side of the picture presented by the phenomenon of neoteny. Hair is vestigial on the body of man, as are the molars, especially the last, which are often not cut at all. The bones are vestigial in the skull of the axolotl, and the maxillary bone, which is present in the newts that undergo metamorphosis into the terrestrial adult form, is absent in those urodeles that are permanently neotenous.

A very instructive case of retardation is that of the bands on snail shells investigated by Diver. These bands vary considerably in the time in ontogeny at which they make their appearance on the shell. Some may be so late as to appear as a mere dot just behind the terminal lip of the shell. 'Others may even be, as it were, pushed off the shell altogether, that is they fail to appear at all because their time of appearance has been so retarded in relation to general development that growth is terminated before they are due.'

It is possible to interpret these cases in terms of the phenomenon of intersexuality in the gipsy moth by noting that, in a normal female moth, the male-producing genes are too slow to produce the appearance of any male characters before the time of maturity arrives and development ceases. Characters may become vestigial by the over-retardation of the rate of action of the factors which control them. The effects of such heterochrony will not in themselves be important in phylogeny, but they will be associated with neoteny, and this, as we have seen, may and often does have important results in evolution.

-99-

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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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