34 THE PALEONTOLOGIC RECORD 



one species is distinguished from another, but they are of generic, 

 ordinal and even class value, and they may be of immense age in the 

 race and mark no special, narrow stage of its history. 



It is a question of interpretation whether each particular phase of 

 expression of fluctuating characters is a matter of time or of environ- 

 ment. 



I have reached the conclusion that it is those species which have 

 the greater degree of normal and persistent fluctuation of character 

 which migrate and follow the shifting conditions of environment, and 

 their life period is correspondingly longer. 



On the other hand species whose plasticity of characters is narrow, 

 are more closely adjusted to their environment, are local in their range 

 of habitat, and temporary in their geological life-period. 



Interpreting the facts on this basis it is the phases of continuously 

 fluctuating characters in species of wide geographic distribution and 

 long geologic range which furnish the most satisfactory evidence of 

 temporary stages in the life history of faunas. 



Another question of interpretation arises when we attempt to re- 

 construct the physical condition of the environment at successive 

 stages of time. 



In a single vertical section we have positive evidence of succession 

 in time. If we were sure that no recurrence of the same fauna could 

 take place we could correlate two vertical sections strictly upon the 

 fauna contained in the strata, on the basis of the supposition that the 

 single fauna appeared but once in the section and that when it ceased 

 in a given section its whole life period was expressed. But the facts 

 show us that this is not the case in nature. In geological times as in 

 the present, we know that many distinct faunas are living on the face 

 of the earth at the same- time, even for very similar conditions of en- 

 vironment. It becomes therefore a very complex matter to correlate 

 two sections in which the order of faunas and the character of the 

 sediments differ; which is generally the case for any two sections sepa- 

 rated by fifty miles from each other, although on stratigraphic evi- 

 dence they may be properly interpreted as covering the same interval 

 of time. 



PALEONTOLOGIC EVIDENCES OF ADAPTIVE EADIATION 



BY PROFESSOR HENRY FAIRPIELD OSBORN 



AMERICAN MUSEUM OF NATURAL HISTORY 



rjlHE law of adaptive radiation 1 is an application of paleontology 

 -- of the idea of divergent evolution as conceived and developed 

 successively in the studies of Lamarck, Darwin, Huxley and Cope. It 



1 Osborn, H. F., " The Law of Adaptive Radiation," Amer. Naturalist, Vol. 

 XXXVI., No. 425, pp. 353-363. 



