456 M. O'Connell — Orthogenetic Development 



We will not go so far, I say, as to agree with Professor 

 Morgan that the vertebrate paleontologist could juggle 

 his specimens and arrange them to show anything he 

 desired to, but we must admit, I think, that there are 

 many chances to go wrong when one makes np a genetic 

 series, say from a dozen adult specimens occurring from 

 Eocenic.time to the Pleistocenic, and that even if these 

 dozen individuals show certain definite trends in develop- 

 ment such as the increase in length of a horn, the loss 

 of a toe, the addition of cusps on the teeth, there will al- 

 ways remain the question: are not these dozen speci- 

 mens simply individuals belonging to many lines, some 

 of which may even have become extinct, and do they not 

 give a general picture of the kind of development of the 

 race as a whole rather than a genetic series? For 

 instance, Professor Osborn has pointed out that in the 

 titanotheres the actual continuity of the series "is broken 

 by the extinction of one branch and survival of another. 

 It is a continuity of character rather than of lines of 

 descent. ' ,9 



If we turn to the fourth field of orthogenetic study, 

 that of invertebrate paleontology, we find that both 

 phylogeny and ontogeny are available in most cases. It 

 is true that the ontogeny of the Crustacea and certain 

 smaller groups is nearly as difficult to study as that of 

 the vertebrates, because the individuals do not retain the 

 record of their early developmental stages, but in the 

 molluscs, corals and brachiopods, the shell or other hard 

 structure preserves the complete record of the life his- 

 tory of the individual from the earliest epembryonic 

 stage to the time of the death of the animal, which was 

 normally in late maturity or old age. In many cases, 

 where the material is well preserved, one may even see 

 the last embryonic stage, so that one may enter, if only 

 a short way, into the field of the embryologist. The 

 invertebrate paleontologist is also fortunate in having 

 large numbers of individuals of a single species to study. 

 He may collect literally a thousand specimens of one 

 species at a given spot and horizon, and then he may col- 

 lect at successive horizons, going upward inch by inch 

 collecting quantities in each stratum and thus he may 

 work out with a certainty hardly to be questioned what 

 was the nature of the changes which took place in that 



9 Osborn, H. F. : Origin and Evolution of Life, p. 264. 



