33a THE SCIENTIFIC MONTHLY
coDsiderable passage of time; they take a definite direction {Muta- tionsrichtung) and reptesent a true evolution of the chromatin. Thia law of definitely directed evolution is illustrated in the detailed struc- ture of the type series of ammonites (Fig. 16) in which Waagen's dis- covery was made. It has proved to be a fundamental law of the evolu-
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��COLLECTIVART- A. SUBRADIATUS F:o. 18. Thb T»pi Bmibh of thk UurArioKs or Waaobh (1869) ik Amuo- KiTBS. BucMsalie matatlcn* of AnmiMllet jiibradlatM drawn and reacnmged Irom tbe orlBloal plat«a at WaBse"-
tion of form, for it is observed alike in InvertebrateB and vertebrates whereier a closely successive series can be obtained. In the inverte- brates a mutation series of the brachiopod, Spirifer miteronatus of the Middle Devonian or Hamilton time, is one of the most typical (Fig. 17). The essential principle of Waagen's discovery, which is one of the most imiwrtant in the whole history of biology, is that certain new char-
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