88 GEOLOGICAL BIOLOGY. 



find that the systematists, studying the structure, have divided 

 the genera into 13 families, grouped in two divisions, Tetra- 

 coralla (2), Hexacoralla (11), and the curves for each of these 

 is separate. Thus we find that the curve of differentiation 

 for the genera of the Favositidae (curve ff of the diagram), 

 which begins with the Ordovician and ends with the Paleozoic, 

 accounts for the main features of the Paleozoic part of the 

 differentiation of the whole suborder. Although other families 

 have their beginnings in the Paleozoic, it is only with a few 

 genera. 



If we examine the curve for the genera of the family As- 

 traeidae {aa on the diagram) it is evident that the chief dif- 

 ferentiation for the early Mesozoic was within this family. 

 This family and the Fungida^ will nearly fill out the total 

 differentiation-curve. The third irregularity in the curve is 

 again explained by the late culmination and differentiation of 

 the family Turbinolidae, bb' , which shows its first genus in the 

 early Mesozoic (Lias), but presents 17 new genera as late as 

 the Tertiary, 



Chronological Value of Family Groups of Genera. — Thus it 

 appears that groups of genera are not only families according 

 to the taxonomist (that is, genera naturally grouped together 

 because of the likeness of their general structure), but the 

 genera composing them are naturally associated together by 

 the time of their initiation among the organisms of the world ; 

 and the simple tabulation of the time-relations of the genera 

 of an order reveals, by the irregularities of the curve of differ- 

 entiation, that the order is made up of several families having 

 separate evolution-curves or separate life-histories. 



The Life-period of a Genus. — The numbers thus given do not 

 refer to the same genera repeated, but in large measure to 

 different genera for each system. Without going into details, 

 this may be illustrated by the following statement : Of the 

 genera above tabulated 182 are peculiar to a single geological 

 system, 89 are found in only two contiguous systems, 40 have 

 a range of three systems, and only 9 range through four sys- 

 tems ; or, to express the fact in proportionate numbers, the 

 life-period, or geological range, of f of the known 448 genera 

 of Stony-corals is not greater than that of a single geological 



