August 14, 1914] 



SCIENCE 



233 



wMcli vitiates in common all calculations 

 based upon the assumption of their constancy. 



In working upon the numerous phyla of 

 vertebrate animals, especially of mammals 

 whose evolution is recorded in our Western 

 Tertiaries, I have been impressed with the fact 

 that they seem to have a fairly constant maxi- 

 Inum rate of progressive evolution. The rate 

 of alteration in structures that are being 

 changed adaptively to some changing environ- 

 ment or habit is fairly uniform, comparing 

 one phylum with another. Where concen- 

 trated upon one element of change or a few, 

 it is more rapid ; when distributed into a great 

 number of alterations of a complex structure 

 it is slow. Some structures are much slower to 

 change than others — notably this is true of the 

 teeth as compared with the bones of the skele- 

 ton. 



It is essentially a constant progressive 

 change. Where we find sudden jumps of any 

 considerable magnitude the explanation is 

 always at hand, and usually obvious when the 

 circumstances are studied judicially, that we 

 are dealing with an imperfect record, and the 

 breaks are due to migration or to unrecorded 

 lapse of time. To prove this point — a disputed 

 one, I am well aware — ^would take me too far 

 afield. I must rest on the assertion that 

 twenty years study, in field and laboratory, of 

 American fossil mammals, has brought me to 

 the conclusion that the evolution of their phyla 

 took place through the cumulation of minute 

 increments of structural change, at a rate 

 which, whether concentrated upon one feature 

 or distributed over many, presents some ap- 

 proach to a uniform maximum. 



I fuUy believe that the change is due to the 

 pressure of the environment, acting through 

 selection upon individual variations. Whether 

 these be mendelian or fluctuative in their law 

 of transmission is iimnaterial. The point is 

 that they are minute, well within the limits of 

 a species as conservative paleontologists draw 

 those limits. 



If they are accumulated through selective 

 action of the. environment, how can they be 

 said to be in any sense free from the varying 

 rate of change of inorganic activities which 



vitiate calculations based upon the constancy 

 of their action. If the environment is chang- 

 ing rapidly at one time, slowly at another, 

 will not this be reflected in the rate of change 

 of any phylum of living beings? Undeniably 

 this is true. Yet there does appear to be a 

 maximum rate of change as above outlined, 

 and environmental change exceeding that 

 limit results in migration and extinction, not 

 in structural alteration. Moreover, a large 

 part of the structural evolution which we can 

 observe must be in reaction to the pressure of 

 the biotie, not the physical environment. A. 

 large portion of the progressive structural 

 change is advantageous to the animal under 

 any circumstances, whether or not the physical 

 environment changes. This is peculiarly true 

 of increase in brain capacity; it is partly true 

 of increase in mechanical perfection of the 

 structure leading to increased speed, better 

 tooth mechanism as well as numerous changes 

 not recorded in the skeleton. 



It would seem therefore that there is a 

 maximum rate at which alterations in the 

 structure can take place. I suppose this rate 

 to be conditioned by two factors, individu'al 

 variability in the organism, and selective proc- 

 esses under the conditions obtaining in nature. 

 At all events the fact stands as of record, 

 proved and confirmed by innumerable in- 

 stances, that the evolution of any direct phylum 

 does take place through cumulation of minute 

 changes, at a rate which, allowing for con- 

 centration upon one element of change or dis- 

 persal over many, does present a considerable 

 degree of uniformity in corresponding parts, 

 whether of the same phylum at different times 

 or of different phyla at the same time. This 

 rate may often not be attained, but I can find 

 no convincing evidence that it can be exceeded. 



The amount, variety and fundamental char- 

 acter of the differences thus accumulated are 

 the practical measure of our systematic classi- 

 fication. A difference or group of differences 

 of small amount, yet distinctly beyond the 

 limits of individual variation, is customarily 

 regarded as specific. Differences of a decidedly 

 larger order are considered generic, and so on. 

 It would perhaps be a fair average estimate 



