CHAPTER XVI 



VARIATION IN TIME 



Two ways of studying evolution in time Approximate method 

 Method of actual evolution The series of forms or phyletic 

 ramifications Polyphyletic genera Discontinuous series Vary- 

 ing rate of evolution of branches. 



PAL^EONTOLOGICAL evolution, that is, the trans- 

 formation of animal forms through the series of 

 ages of the earth, of all evidence constitutes the 

 most direct and the most demonstrative proof of 

 the transformist hypothesis. Rather neglected 

 by the creators of the descent theory, chiefly on 

 account of the scarcity of documents, it has become, 

 on the contrary, in the last thirty years of the 

 nineteenth century, the principal object of the 

 efforts of modern palaeontologists. 



The study of the changes of fossil animals has 

 been approached in several ways. The most 

 logical, and, at the same time, the most exact, is 

 the patient and close reconstitution of the gradual 

 series of forms through which a given branch of 

 the animal kingdom has passed when rising from 

 one geological stage to another, and even, if possible, 

 from one stratum to another of the same stage. 

 With this method we connect the names of Waagen, 

 Neumayr, von Zittel, Hyatt, Mojsisovics, Osborn, 

 Schlosser, Stehlin, etc. 



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