THE PALEONTOLOGIC RECORD 77 
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 RADIATION 
By Prormssor HENRY FAIRFIELD OSBORN 
AMERICAN MUSEUM OF NATURAL HISTORY 
Hi law of adaptive radiation’ 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 
*Osborn, H. F., “The Law of Adaptive Radiation,” Amer. Naturalist. Vol. 
XXXVI, No. 425, pp. 353-363. , 
