As species are produced and exterminated by slowly acting and still existing causes, and not by miraculous acts of creation; and as the most important of all causes of organic change is one which is almost independent of altered and perhaps suddenly altered physical conditions, namely, the mutual relation of organism to organism-the improvement of one organism entailing the improvement or the extermination of others; it follows, that the amount of organic change in the fossils of consecutive formations probably serves as a fair measure of the relative, though not actual lapse of time.
Espezieak, astiro diharduten eta egun ere badiren kausen eraginez, eta ez kreazio-egintza mirakulutsuz, sortu eta iraungitzen direnez, eta aldaketa organikoen kausarik inportanteena baldintza fisikoen aldaketarekin, eta agian bat-bateko aldaketarekin ere, zerikusirik ez duena, hau da, organismoak organismoarekin duen harremana denez gero, organismo baten hobekuntzak bestearen hobekuntza edo suntsiketa dakarrelarik, hemendik ateratzen da jarraikako formazioetako fosilen aldaketa kantitateak ziurrenik balio duela igaroriko denbora erlatiboaren neurria emateko, baina ez absolutuarena.