of the Costce in the Peris phinctince. . 457 



species at each level, that is, what varieties arose (in the 

 Waagen sense), and he may trace also the changes in 

 time from level to level, that is the mntations. 



In addition to these geographic and geologic data in 

 phylogeny he has the entire life history of each individ- 

 ual so that he may pass from the details of ontogeny 

 to the generalities of phylogeny in accordance with the 

 law of recapitulation. But this law of recapitulation 

 has to the invertebrate paleontologist a very different 

 meaning from that which it has to the embryologist and 

 zoologist as Hyatt, Jackson, Smith, Grabau and others 

 have shown. The embryologist finds that there is a gen- 

 eral similarity between the adults of certain lower forms 

 and the embryonic stages of higher forms in living 

 organisms, but the invertebrate paleontologist compares 

 the entire epembryonic development or the stages 

 between the embryo and the adult step by step with the 

 adults in earlier geologic horizons, finding in the life 

 history of a single individual an epitome not alone of the 

 development of the genus, but of the entire phylum. 

 These differences in the viewpoint of the law of recapitu- 

 lation are well known, I merely repeat them here because 

 as we understand recapitulation so do we understand 

 orthogenesis. The invertebrate paleontologist ever ap- 

 proaches ortho-phylogeny in the light and under the 

 guidance of ortho-ontogeny, dealing with the minute 

 changes in the life history of the individual and at most 

 arguing for orthogenetic development in a single phylum, 

 as in corals or ammonites, but never invoking it to cover 

 the method of development from one phylum into another. 



Of all the invertebrates the ammonites offer, perhaps, 

 the best opportunities for orthogenetic studies because 

 of their abundance, their comparatively rapid evolution, 

 their complexity in structure, involving as it does a large 

 number of variable characters, and their mode of growth. 

 Because of the coiling of the ammonite shell the early 

 stages are always accessible in well-preserved material, 

 and by separating off the successively earlier and earlier 

 whorls one may ascertain what were the various steps 

 in the life history of any single individual. One may 

 thus follow out the entire ontogeny and from this one 

 may, with reasonable certainty, predict what was the 

 phylogeny of the genus or family to which the species 

 under study belonged. 



