RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 295 



characterize the respective stages and substages of the geological scale. 

 While the principal evidence of the full chronological order of the 

 stages and substages of the scale lias been derived from results of pro- 

 gressive evolution alone, the results of the empirical studies just men 

 tioned are of the utmost importance in systematic geology as well as in 

 all practical geological investigations. Indeed, not only the first steps 

 in the construction of the geological scale, but the working out of all its 

 details, are the result of empirical study, while the result of the philo- 

 sophical study of all its fossil forms followed and completed it as a 

 chronological standard. 



(4) The average rate of progessive evolution for the different branches or divi- 

 sions of both the animal and vegetable kingdoms has not been the same for each in 

 all parts of the world, nor the same for all in any one part «>l the world, during all 

 the time they have coexisted. 



While the various divisions of geological time as expressed in the 

 construction of the great geological scale aresatisfaetorilyirecognizable 

 by their respective fossil faunas and floras, each as a whole, their limits 

 are often obscured not only by the ranging of certain specific forms 

 from one to another, but by the relative acceleration and retardation of 

 the rate of progressive evolution of certain of the types which are dis- 

 tinctive of the divisions of the scale. Such retardation and accelera- 

 tion have occurred in various divisions of both the animal and vegeta- 

 ble kingdoms, which has not only resulted in obscuring the limits of 

 the recognized divisions of the scale, but in imparing to some extent 

 the relative chronological value of the characteristics possessed by 

 fossil remains belonging to different branches respectively of the animal 

 and vegetable kingdoms. 



These differences in rate were no doubt largely due to inherent differ- 

 ences between those great groups of organic forms respectively, but they 

 were also largely due to differences in the effects of the same environing 

 conditions upon different groups; that is, the conditions which were 

 congenial to the existence of marine, fresh water, and land faunas and 

 land floras, respectively, <>r. in short, the conditions under which marine 

 and continental life, each as a whole existed, have been so different and 

 in many cases so incongruous that their relative rate of advancement 

 in progressive evolution was necessarily unequal even under similar 

 climatic and hydrographic conditions, and much more unequal when 

 these conditions were different. 



For example, in Europe a certain progressive grade was reached for 

 the whole of animal and vegetable lite which all geologists recognize 

 as Cretaceous. In Xorth America remains of invertebrate life, and in 

 part those of vertebrate life, exhibit evidence of essentially the same 

 Cretaceous grade, but associated remains of vegetable life show a much 

 more advanced grade, while a few vertebrate types show an earlier or 

 retarded grade, all being judged by the European standard. 



The foregoing remarks apply particularly to the first part oi propo- 



