24 OEGANIC DEPENDENCE AND DISEASE 



Christian era. It would be only a long guess to tell why 

 Leptaena rJiomhoidalis lived long and was more quickly 

 adaptive than others of its congeneric associates. Not a 

 feature of structure observed or deducible points to the ex- 

 planation. Another brachiopod, Atrypa reticularis, lived 

 through the millions of years from the Silurian into the 

 Carboniferous with but indifferent modifications of its 

 specific type. Some paleontologists may say that these 

 statements fail to recognize the chronologic differences in 

 these stabilized types, and that to identify living Forami- 

 nifera, for instance, with those of the Mesozoic and of the 

 Silurian is hasty and incompetent. It is an a priori state- 

 ment without demonstration. For the brachiopods at least, 

 the Lingulas, the Leptaenas, the Atrypas, the fact remains 

 after careful scrutiny that the differences have not proved 

 permanently translatable in terms of time and change, are 

 hence negligible, and that other distinctive generic names 

 that have been applied to them are not of much account. 

 These are long-lived creatures, and, while exceptional in 

 their longevity, we must try to realize that by virtue of 

 structural and functional constitution they acquired an ad- 

 justment or resistance to change which made them as nearly 

 permanent and as completely stabilized as life, it would 

 seem, can ever become. Their endurance without change 

 can be expressed only in millions of years. Armored or 

 protected, they were the more competent for this long life. 

 But even Methuselah died, and Leptaena rhomhoidalis died 

 at last, as a species, through some revolutionary malad- 

 justment which would no longer permit its endurance. 

 These are patriarchal life periods; but for the multitude 

 of species of the past that have kept their characters un- 

 altered through a single geological system or a major sub- 

 division of it, we must think of their days in thousands of 

 thousands of years and not in any terms of easy concep- 

 tion that we might use in our conventional expressions of 

 time. 



