22 HYATT ON THE TERTIABY SPECIES 



It is impossible to construct a series and begin to investigate the causes of the origin 

 of the forms without assuming continuity of descent, and the action of time and changed 

 conditions in modifying the organization, as has been done by modem experimental 

 zoologists. 



If this position is the true one, then similar physical causes acting through similar 

 periods ia time, upon the same or different genetic series of animals, ought to produce 

 results or modifications in which not only the action of time and the environment upon 

 the animal, but also the reaction of the laws of heredity and growth, would be distinctly 

 manifested. This has seemed to me to be the case among the Steinheim series, and among 

 the Ammonites, and to account for the sudden appearance of geratologous resemblances, 

 arrests of development and reversions, all of which are pathological ; and due, like other 

 pathological conditions, to unfavorable surroundings. 



There are two extreme classes of cases which might be considered exceptions to such 

 a law, one class embraces what paleontologists call persistent types, and another those 

 curious parasites, which like the Epizoa and others among Crustacea, or the parasitic 

 Vermes lose in the adult a portion or nearly all of their typical characteristics. 



The persistent types are such animals and their fossils, as ia Lingula, Nautilus ; 

 Myrmecobius among mammals ; Ceratodus among fishes ; and a host of other forms, 

 which exhibit at the present day very nearly the same forms as those of the same genus 

 found in Paleozoic or Mesozoic time. Even if this statement be doubted, as it may 

 reasonably be with regard at any rate to the LinguljB, as stated by Mr. Dall, and with 

 the Nautili, there still remains the fact that these types are persistent, or do not 

 present any modifications of their organization at all proportionate to the changes through 

 which they passed. Paleontologists have noted these peculiar and remarkable instances, 

 but failed to call attention to the fact that many groups present a greater or less number 

 of species which can be classed in the same category with these more noted examples, and 

 that, after all, this is not an uncommon phenomenon. 



Almost every group of Ammonites contains such species, and I have tried to show in 

 previous publications that all of these persistent species or forms were among the lower, 

 or earlier occurring, members, or more embryonic forms of the groups to which they 

 belonged. When taken in conjunction with the fact that none of the extremely per- 

 sistent forms exhibited geratologous transformations, these facts appeared to show that 

 the reason why time and changing conditions had no more effect, was due to the 

 enormous power of reaction in the organism itself, its growth force, which enabled it 

 to withstand the action of the shifting environment, and to adjust itself to these changes 

 without materially modifying its own organization. Such a case is also presented here, 

 and it is PI ^uZ% P^- oxystomus, and Fl Steinheimensis which have the greatest range in 

 time, and are found in all the formations. 



If it be true that growth force has anything to do with the life of a series, as it has 

 to do with the life of an individual, then there ought to be some common ratio between 

 the power of reproduction in the series and in the individual, and between the hfe 

 power of these persistent types and the point at which they sprang from the ancestral 

 tree. In fine, if growth force has any meaning at all, and has, as is here claimed, an 

 influence upon the life of a series in the same way that gravitation acts upon the 



