24 
ORGANIC DEPENDENCE AND DISEASE 
Christian era. It would be only a long gness to tell why 
Leptaena rhomboidalis 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 bracliiopods 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 rhomboidalis 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. 
