86 GEOLOGICAL BIOLOGY. 



right successively, representing the successive geological eras, 

 Cambrian, Ordovician, Silurian, Devonian, etc. The name of 

 the era is indicated at the top of each column by its initial let- 

 ter. The length of time of each of these eras is represented 

 roughly by the width of the spaces between the separating 

 lines, according to the time-scale described in Chapter III. 

 Thus, starting from the lower left-hand corner, the abscissa 

 represent time-extension from the beginning of the Cambrian 

 era. 



Drawing an horizontal line from this point across the base 

 of the several columns, the distance above this base-line or 

 the ordinate expresses the degree of differentiation in terms of 

 units of genera (or of species, as the case may be) appearing in 

 each era. 



Thus, by connecting together the points representing the 

 amount of differentiation (the ordinate) for each geological 

 era (the abscissa), we produce a curve representing the rate 

 of generic differentiation for the particular order or class 

 (as the case may be) under consideration. This curve may be 

 called the evolution curve. In the following table are repre- 

 sented the evolution curves of the Madreporaria, and those of 

 several divisions and families of the Madreporaria, based upon 

 the statistics before us. 



Construction of the Diagram. This diagram was constructed as follows: 



Extension laterally represents time - duration, beginning at the left-hand 

 lower corner with the base of the Cambrian; the total length of geological time 

 thence to the present is made to cover 100 spaces. The several geological 

 system-eras are represented in their estimated proportionate lengths, thus: 

 15$ is given to the Cambrian, io# to the Ordovician, 10$ to the Silurian, 15^ 

 to the Devonian, and 15^ to the Carboniferous; to the Triassic 5*, Jurassic 5*, 

 Cretaceous io#; and the Tertiary and Quaternary together are given 15$, 

 io# to the former and 5$ to the latter. 



This, it will be seen, assigns to the Paleozoic, Mesozoic, and Cenozoic,. 

 respectively, 65^, 20$, and 15*, or 13, 4, 3 as the time-ratios, Dana's revised 

 estimate (1895) being 12, 3, I, and Walcott's estimate stands 12, 5, 2, as ex- 

 plained in the third chapter. 



Vertical lines are drawn to separate off the time-scale into periods with these 

 proportions. Vertical extension of the curved lines represents the number of 

 new genera of each period. The curve m running highest is the curve of generic 

 differentiation for the Order Madreporaria, and is compiled from the lists of 

 genera in Zittel's "Handbuch," with some corrections based upon facts appearing 

 since its publication, and the geological range there assigned to them, with a 

 rearrangement of the genera of the first family of the Hexacoralla, Favosi- 

 tidae, into Favositidae and Poritidae. 



