July 28, 1905.] 



SCIENCE. 



109 



vicissitudes just mentioned. Moreover, the 

 most ancient types found among living 

 fishes are denizens of fresh, and not of 

 marine, waters. This fact indicates that 

 the genetic lines of those iehthyic types also 

 were not wholly broken by such shifting 

 physical conditions, and that the genetic 

 lines of their marine congeners were not 

 preserved by having less restricted and 

 more uniform conditions of environment. 



The chief object of the immediately pre- 

 ceding paragraph is to show the strong 

 contrast between the conditions which have 

 attended the genetic descent of fresh-water 

 and marine faunas respectively. By way 

 of explanation it is proper to say, however, 

 that the only manner in which unbroken 

 genetic lines of fresh-water faunas could 

 have been preserved through the geological 

 ages to the present time is through un- 

 broken fresh water conditions. Such con- 

 ditions could have been preserved only by 

 the persistence of rivers in their established 

 channels, so that at least some portions of 

 them, together with their faunas, have es- 

 caped des.truction by all subsequent eleva- 

 tions and depressions of land surface.^ 



During the Triassic and Jurassic periods 

 the genus Unio was the only known repre- 

 sentative of its family, and it has always 

 been the leading genus of that family since 

 those periods. This fact shows that, con- 

 trary to a prevailing belief, rare genera are 

 not always disappearing or decadent gen- 

 era. It is also worthy of remark in this 

 connection that although genera were some- 

 times so long-lived, the geological record 

 shows that the duration of species was 

 comparatively short. The sudden intro- 

 duction of the Unionidge by its leading 

 genus, Unio, without known congeners, and 

 its survival unchanged through all subse- 

 quent ages under peculiarly adverse condi- 



^ See my discussion of this subject in Third An- 

 nual Report of the U. S. Geological Survey, pp. 

 479-486. 



tions, are all inconsistent with the theory of 

 the origin of species by natural selection. 

 They are also inconsistent with the assump- 

 tion that the influence of physical condi- 

 tions of environment has been a constant 

 dominant factor in phylogenetic differen- 

 tiation, on the one hand, and of conserva- 

 tion of established living forms on the 

 other. 



The earliest known remains of the great 

 subclass of dinosaurian reptiles are found 

 in early Triassic strata, and the latest 

 known representatives of that great sub- 

 class barely survived the close of the Cre- 

 taceous period, as is shown by the pres- 

 ence of their remains in the upper strata 

 of the Laramie formation, associated with 

 animal and plant remains belonging to 

 Tertiary types. The beginning and end of 

 the comparatively short time range for 

 such a large and diversified subclass of 

 dominant and distinctive animals is shown 

 by the right-hand line under D on the 

 diagram. Those strangely peculiar ani- 

 mals were introduced suddenly, soon ex- 

 isted in multitudes, became dispersed over 

 the earth with great rapidity and, from 

 their beginning, they were the dominant 

 animals of all the continents. They varied 

 in size from that of a rabbit to that which 

 would be equal to several large elephants; 

 and the grade of organization for the whole 

 subclass was as high in the earlier as in the 

 later part of its existence. They were dif- 

 ferentiated into flesh eaters and plant eat- 

 ers and into denizens of land and water 

 respectively. We know absolutely noth- 

 ing of the genetic origin of those remark- 

 able animals, and no traces of similar ani- 

 mals have been found in any strata older 

 than those containing their early Triassic 

 remains. Their world-wide decadence was 

 not delayed by the improving earth-condi- 

 tions which the mammalia, soon to assume 

 faunal dominion, found abundantly con- 



