RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 309 



fresh-water, faunas. Among illustrations of this tardiness of evolution 

 may be mentioned the close similarity to one another of the Jurassic, 

 ( Jretaceous, Tertiary, and recent fresh-water mollusean faunas, and also 

 the fad that much the greater pari of the ancient ichthyic types which 

 have survived to the present day arc found among the fresh- water fishes. 



The foregoing comparisons have been made between marine faunas 

 (.11 the one hand and all other faunas and the land floras on the other. 

 When we come to compare the various kinds of land animals with one 

 another, and with land plants, we find that while reciprocal biological 

 relation between them is more or less intimate in certain respects, there 

 is no such relation as would necessarily have produced a concurrent 

 rate of progressive evolution in all of them. On the contrary, when we 

 come to examine the fossil land faunas and floras we tin.l that there has 

 been a great difference among them as to the rate of progressive evo- 

 lution for each, and also a marked difference in the relative extent of 

 differential evolution. 



For example, the earliest known dinosaurs were introduced before 

 the earliest known dicotyledonous plants. The former, after a wonder- 

 fully differentiated development, became extinct, together with tin' 

 enaliosaurs, about the time of the introduction of the earliest known 

 of the placental mammals. Other reptilian families, and even closely 

 related genera, which were contemporary with the dinosaurs and enal- 

 iosaurs, have survived to the present day. Long before the extinction 

 of the dinosaurs, and before the introduction of the earliest known pla- 

 cental mammals, a dicotyledonous flora prevailed, composed largely of 

 families which are well represented by living plants, several of which 

 families contain genera that are common among living floras.* 



Up to the close of the Cretaceous the general rate of progressive 

 evolution of land plants was, as shown in the preceding essay, more 

 rapid for North America than for Europe: but it was afterward much 

 less rapid on this continent than was that of the contemporary placen- 

 tal mammals. 



For the time that the dinosaurs are known to have existed their rate 

 of progressive evolution was very slow and their differential evolution 

 very great. That is. while their differential evolution resulted in an 

 infinite variety of forms and their adaptation to the greatest extremes 

 in methods of locomotion and of dietetic subsistence, the average bio 

 logical rank of the Jurassic and Cretaceous dinosaurs seems not to have 

 been appreciably higher than is that of their Triassic predecessors. 

 Indeed, it is an admitted fact that many of the latest known North 

 American dinosaurs have strong Jurassic affinities; and it may also be 

 remarked that the affinities of their associated nonplacental mamma 

 ban remains are similar in this respect. 



*AI1 these comparisons are made wiih special reference o> North American paleoii 

 tology. 



