234 



SCIENCE 



[N. S. Vol. XL. No. 1024 



to say that one genus differs from the next 

 ten times as widely or fundamentally as one 

 species from its next neighbor. There is no 

 sort of exact rule in the matter, but this would 

 perhaps represent the average opinion to which 

 each systematist endeavors to conform in 

 arranging the group upon which he is working. 

 Now if the above conclusions are warranted 

 we may find in the recorded evolution of vari- 

 ous weU-known phyla a rough measure of the 

 relative length of the epochs covered by its 

 evolution. In instance we may take the evolu- 

 tion of the horse. This phylum as represented 

 in the American Tertiaries I believe to be a 

 direct phylum so far as the genera are con- 

 cerned; the relation of the species to the 

 direct line of descent are mostly immaterial to 

 the present discussion. 



Relative 

 Amount 

 of Structural Dif- 

 Equidae ference from Geologic 



(Direct Phylum) Preceding Stage Epochs 



Equus caballus, etc 1 Recent 



Equus scotti, ete 10 Pleistocene 



Hipparion 10 Pliocene 



Meryehippus 15 Miocene 



Farahippus 5 



^io^iPP^ ^5 oiigocene 



.'.'.'..'.".'.'.'.10 



10 Eocene 



PaJeocene 



It would be possible to verify these estimates 

 of structural differences by comparative meas- 

 urements. But it would be an enormous task. 

 To select a few of the great number of struc- 

 tural differences for measurement would be 

 almost certainly misleading; to average them 

 all would entail many thousands of measure- 

 ments for each species or genus compared. 

 The final result might be twice as much or 

 half as much as the estimate I have given ; it 

 would certainly not be ten times or one tenth 

 as great. The margin of error for each esti- 

 mate here given is not to any great extent 

 cumulative for the whole series. The errors 

 would therefore tend to balance to some extent, 

 and the margin of error for the whole series 

 would be less in proportion. For these rea- 

 sons, and because of the doubt already ex- 



pressed as to whether the maximum rate of 

 evolution is really a constant, I have not 

 thought it worth while to verify the estimates 

 by measurements. 



From the beginning of the Pleistocene to 

 the present time, the evolutionary change in 

 the phylum is measured by the difference be- 

 tween the modern species and the nearly allied 

 species found in the Aftonian and other equiv- 

 alent formations of early interglacial time. 

 During the Pleistocene there has been a great 

 deal of migration and shifting of faunas; the 

 actual evolutionary change in this or any 

 other mammalian phylum is notably small. It 

 is perhaps one tenth the amount of structural 

 change that separates Equus from Hipparion 

 of the late Miocene and early Pliocene. Hip- 

 parion in turn differs about as much from 

 Meryehippus as it does from Equus; the esti- 

 mated structural difference between the earlier 

 stages is represented by the remaining figures 

 in the column. Adding up these figures, we 

 find that the amount of structural change in 

 the Equus phylum during the Tertiary is 85 ^c 

 times the amount of Pleistocene evolution. 

 So far as this is a measure of geologic time, it 

 means that the Tertiary from Suessonian up- 

 ward was 85 times as long a period as tEe 

 Pleistocene. To this should be added a con- 

 siderable figure for the Paleocene, whose 

 length based on the evolution of other phyla 

 might be assumed at 10 or 15 times the length 

 of the Pleistocene. Briefly then, on this basis 

 we should assume that the entire Tertiary is 

 about 100 times as long as the Pleistocene, 

 dating the latter from the first great glacial 

 advance. 



This is greatly in excess of the proportion 

 usually assigned. But the Pleistocene was a 

 time of extreme activity in sedimentation, de- 

 nudation and other inorganic activities whose 

 rate affords the basis of the various calcula- 

 tions that have been made. The amount of 

 Pleistocene denudation, the thickness of its 

 sediments, would hence give a greatly exag- 

 gerated measure of its length in time as com- 

 pared with the whole of the Cenozoic. 



,The various other phyla of mammals sup- 

 port these proportions fairly closely. None are 



