Page:Popular Science Monthly Volume 77.djvu/83

Rh one species is distinguished from another, hilt 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 environment.

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 reconstruct 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 environment. 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 separated by fifty miles from each other, although on stratigraphic evidence they may be properly interpreted as covering the same interval of time.



HE law of adaptive radiation is an application of paleontology J-of the idea of divergent evolution as conceived and developed successively in the studies of Lamarck, Darwin, Huxley and Cope. It