quality what they lack in quantity and are correspondingly better developed.

Be that as it may, the retention in the adult descendant of a character which was juvenile in the ancestor is an example of neoteny.


Neoteny in plants

It was realized by Nicoloff that the theory of recapitulation could not be generally applied to plants because the structure of young forms is so often found to be useless in the search for the affinities of a species. Guillaumin pointed out that the young foliage leaves of a plant frequently are more specialized than those formed later on, which means that evolutionary novelties make their first appearance in young stages. These observations have been extended by Gaussen and by de Ferré who have shown that in conifers the characters of young stages of plants that are believed to be ancestral are found in the fully formed stages of their descendants. Of the series Pinus, Cedrus, and Abies, Pinus is held to be the most primitive and Abies the most specialized on morphological and palaeontological grounds. The fully formed leaves of Pinus are clustered on dwarf shoots, but the young leaves are single and borne on long shoots and resemble the definitive leaves of Abies. Comparable observations have been made on cones and other structures. These cases where 'the young form shows the way of future evolution' are examples of neoteny.


Conclusion

On looking back over this chapter we may say that there are a number of cases in which the adult structure of an animal resembles the embryonic or larval structure of another animal, and in each case there are good reasons for regarding the former as having been derived from the latter: i.e. the adult descendant resembles the young stage of the ancestor, which is precisely the reverse and opposite of that which would be required under the theory of recapitulation. If the neotenous condition is achieved by accelerating the rate of development of the reproductive glands and hastening the time of maturity, the phylogeny usually results in a simplification often associated with parasitism. But if the neoteny is brought about by a slowing down of the rate of

-90-

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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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