FOSSILS THEIR NATURE AND INTERPRETATION. 8/ 



The differentiation curve is formed by making a vertical scale and placing 

 the point representing the differentiation for each period above the base-line by 

 the number of divisions corresponding to the number of new genera initiated 

 during the period. In the same way separate differentiation-curves are formed 

 for the genera of several of the families: thus / is the curve for the Favositidze; a, 

 the curve for Astraeidae; b, for the family Turbinolidae. 



Paleozic Time 



Ordovician Silurian 



Carboniferous 



in- 



Mesozoic Cenozoic 



U8\ /Trl. Jur. gretaceouk /Tertiary Qy.K 



e 



\J 



m 



FlG. 5. Evolution curves of the families of the Madreporaria. The vertical lines represent the 

 points of time separating the several geological eras of which the names are at the top of the 

 chart. The horizontal lines represent, by tens, the number of new genera first appearing in 

 each era. The curved lines represent the rate of differentiation of each family type in 

 number of genera first appearing in each successive era. mm' evolution curve for the whole 

 Madreporaria, tt' for the Tetracoralla, hh' for the Hexacoralla, ff for the Favositidas, aa f 

 for the Astraeidae, bb' for the Turbinolidae. 



Meaning of these Evolution Curves. This diagram illustrates 

 the following points : The curves express the rate and the de- 

 gree of differentiation of generic form expressed in the subor- 

 der Madreporaria in geological time. This law for the whole 

 group is expressed in curve m. The irregularities of the 

 curve suggest at once that it is compounded of at least three 

 independent curves, of which the nodes are at the close of the 

 Silurian, Jurassic, and Tertiary, and this suggestion is verified 

 by examination of the taxonomic classification. There we 



