454 M. O'Connell — Ortho genetic Development 



passing over as of little or no importance the complete 

 epembryonic development. It is trne that the embryolo- 

 gist sees successive changes in growth and that he can 

 make certain general comparisons, but, as is well known, 

 the history which he reads is so abbreviated, so many 

 chapters are lost through condensation and elimination, 

 that he catches but an evanescent glimpse of this or that 

 ancestral condition as the pages pass rapidly before his 

 gaze. Furthermore, he makes his comparison not so 

 much between entire embryos of various types, as 

 between the different organs of embryos higher or lower 

 in the scale of life. Thus he traces the homologies of the 

 fore-limbs in the embryo of a bird, the bat and man, he 

 finds that the heart in mammals in the course of its 

 embryonic development passes through fish, amphibian 

 and reptilian stages, but in these and all similar cases 

 comparisons are made between stages of development of 

 organs not organisms and the recapitulation observed is 

 so general that we see the same kind of embryonic suc- 

 cession in all mammals, for instance, with variations 

 produced more by what is left out of the history than by 

 what is added, for each organism, if it could give a com- 

 plete recapitulation of its ancestral characters, would 

 have to show all that the paleontologist already knows 

 and all that he hopes to know as well as what he never 

 will know. Therefore, in any given embryo we see a few 

 selected pages, but it is clear that orthogenetic trends 

 will be entirely obscured, since such directions are seen 

 in individuals, in species, in genera but not when we pass 

 from phylum to phylum, from fish to reptile, to amphi- 

 bian, to mammal. 



The third field open for orthogenetic studies is that of 

 vertebrate paleontology. Here one may observe succes- 

 sive changes in time and may work out evolutionary 

 series which in many cases show orthogenesis or, as 

 Professor Grabau has called it, ortho-phylogeny, in con- 

 tradistinction to ortho-ontogeny. As a rule the verte- 

 brate paleontologist deals with adult individuals and 

 for any species he may know nothing of the young and 

 sub-mature stages of development. In other words, he 

 may arrange an entire phyletic series without knowing 

 anything about the ontogeny of the species in that series. 

 A knowledge of the ontogeny must always be fortuitous, 

 depending upon the finding in a single place of a large 



