ORIQIN AND EVOLUTION OF LIFE 331
earth crust of North America'* is witneBsed in fourteen periods of mountain-making of varying importiince. Between these relatively short periods of upheaval came" periods when the continent was more or leSB flooded by the oceans. There are certainly twelve and probably not less than seventeen periods of continental flooding which vary in extent up to the submergence of four million square miles of surface. Each of these changes, which by some geologists are believed to be cyclic, included long epochs especially favorable to certain forms of life, resulting in the majority of cases in high specialization like that of the t*a-scorpions (Euiypterids) followed by more or less sudden extinction. Changes of environment play so large and conspicuous a part in the selection and elimina- tion of the invertebrates that the assertion is often made that environment is the cause of evolution, a Btatement directly contrary to our fundamental biologic law that the cause of evo- . lution lies within the four compleses of action, reac- tion and interaction (see p. 10), Perrin Smith, who has made a most exhaustive analysis of the evolution of the cephalopod molluscs and especially of the Tri- assic ammonites, observes fig. 10. a fossil STAB-naa or devomas that the evolution of form times, associatki, with and devoihino bi-
. , , ,, VALVES. Hamilton group, SaugertlCT, New York.
continues unmterruptedly After John m. curke.
even where there is no
evidence whatever of environmental change.
It was in the ammonites that Waagen first observed the actual mode of transformation of one animal form into another, set forth in his classic paper of 1869, "Die Formenreihe des Ammonites subradia- fiM."" The essential feature of the "mutation of Waagen"** is that it established the law of minute and inconspicuous changes of form which accumulate so gradually that they are obser\'able only after a
- ' Pirsson, Louts V., and Schuchert, Charles, 1915, p. 979.
»« Op. cit., p. 982.
I' Wasgen, W., 1869.
"The term "mnUtion" wbb introduced by Waagen in 1869, Twenty years later the great Austrian paleontologist, Neumayr, defined the " Mutationsrich- tnug" as the tendency of form to evolve in certain definite directiona. Sec Neu- mayr, M., 1889, pp. 60, 61.
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