

CHAPTER XXIV 



INDIVIDUAL AND PALJEONTOLOGICAL EVOLUTION : 

 ONTOGENY AND PHYLOGENY 



The great biogenetic law of Haeckel Embryological acceleration or 

 tachy genesis Embryonic types persisting in the fossil state 

 Study of individual development in the Ammonoids and the 

 Lainellibranchs The milk teeth of Mammals. 



THE law of progress manifests itself quite as plainly 

 in the development of the individual as in that of 

 a group. Omne vivum ex ovo has become a common- 

 place axiom, affirming that the most complicated 

 individuals proceed from an egg, that is, from a 

 monocellular being similar to the lowest types of 

 the animal scale. The species born from the egg 

 subsequently passes through a series of phases of 

 development, more complex and more numerous 

 as the group is higher in the scale. Very early 

 came the idea of comparing the phases of the in- 

 dividual development with those traversed by the 

 group itself in the course of its palaeontological 

 evolution, and of establishing a parallel between 

 these two developments : the first very rapid, the 

 second much slower. The concordance of Onto- 

 geny and Phylogeny has become, in the hands of 

 Geonroy-Saint-Hilaire, ofJSerres, of Miiller, and, 

 above all, of Haeckel, the great biogenetic law, 



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