RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION, 293 



in these essays. Therefore, a certain lack of immediate relevancy will 

 appear in the order in which they are stated. 



I 1 ) All species of animals and plants have originated genetically from preexisting 

 forms, and therefore nil arc more or less mutable as regards their reproduction. These, 

 together with the various divisions higher than species into which the animal and 

 vegetable kingdoms are divisible, have respectively acquired their distinguishing- 

 characteristics hy differential and gradually progressive evolution. The extinction 

 of all species and other divisions of the animal ami vegetable kingdoms which lias 

 taken place during geological time has always been by natural means and in accord- 

 ance with natural laws. It has generally been secular and gradual, but in many 

 cases locally or regionally accidental. No universal extinction has ever occurred. 



This proposition is presented mainly as a countercheck to those por- 

 tions of the preceding series of propositions which assert the special 

 creation and periodically universal extinction of species, and also as a 

 necessary concomitant of the propositions which follow. It is proper in 

 this connection, however, to make some general remarks concerning 

 species. 



It was comparatively easy to define a species in accordance with the 

 views of the early naturalists, but it is more difficult to do so in accord- 

 ance with the principles of modern biology. Because of this, and doubt- 

 less in part because of the lingering influence of those early views, there 

 is much difference of opinion as to what constitutes a species. This is 

 especially observable among those who describe and publish fossil re- 

 mains. Some treat every form which is describably different from 

 another as a distinct species, while others treat these closely similar 

 forms, especially if they evidently belong to the same fauna, as varieties, 

 and apply the term species in a more comprehensive manner. 



I adopt the latter method in these essays, and regard as belonging to 

 one and the same species all assemblages of individual forms, even if 

 they are very variable, which occur in strata of the same stage, or if 

 they occur in adjacent or other stages, which there is reason to believe 

 freely interbred and were capable of producing the same kind with its 

 varieties. — that is, 1 regard species as being variable as well as muta- 

 ble. Still, although species are more or less variable, they have a 

 recognizable entity, for while they are mutable they possess a certain 

 tendency to stability of characteristics which has remained through long- 

 lines of reproduction or until gradually overcome by evolutional change. 



(2) Coincident with the progress of evolution, notwithstanding the retardation, 

 inertion, and even degradation that have occurred along certain lines, there has 

 been during geological time a general average advancement in biological rank of ani- 

 mal and vegetable tonus, evidence of which is afforded \>\ certain characteristics of 

 their fossil remains. The evidence of this general advancement constitutes trie ulti- 

 mate standard of measures of geological time as a whole and the principal means 

 of ascertaining the order of full succession of the events which attended the pro 

 dnction of the stratified rocks of the earth. 



ft is true, as was briefly mentioned in Essay n, tliat the practical 

 geologist finds numerous local indications of the relative age of for- 



