which is typical in fishes, may be explained in this way as Versluys suggested, but there are difficulties in this case.

E. S. Goodrich ( 1913) showed that the position of the fins of fish has been transposed up and down the body in phylogeny, but that there is no shifting during ontogeny. The fin arises in a new position from its first appearance in ontogeny, which shows that there has been no recapitulation; instead, progressive deviation has resulted in the substitution of a new position for that of the ancestor. Sewertzow also presented similar evidence, which he used to illustrate his principle of archallaxis.

The same phenomenon has been shown by Butler to occur in the teeth of mammals. Transitions from the pattern of typical molars to that of the teeth in front of or behind them take the same form in related species, but do not always affect the 'same' teeth ('sameness' means identity of numerical position in the jaw). It follows, therefore, that a particular tooth-type can in phylogeny move backwards or forwards in the jaws. But there is no shifting during ontogeny. The teeth arise in their new position from the start of the ontogenetic development of each individual.


Conclusion

It was pointed out by Sedgwick that 'the evidence we have seems to indicate that evolutionary modification has proceeded by altering and not by superseding: that is to say that each stage in the life-history, as we see it today, has proceeded from a corresponding stage in a former era by the modification of that stage and not by the creation of a new one. . . . The evidence seems to show, not that a stage is added on at the end of the life-history, but only that some of the stages in the life-history are modified.' This view has been substantiated to an ever-increasing extent with the progress of knowledge, and it expresses in succinct form why Haeckel's biogenetic law is rejected. The embryo of the descendant passes through modified stages of the ancestral ontogeny; it does not pass through and beyond the ancestral adult stage.

Kryžanowsky considers all modifications of ontogenies in evolution to have taken the form of deviations and substitutions. He distinguishes between 'autogeneous' and 'heterogeneous' substitution. In the former, existing ontogenetic phases are

-61-

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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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