THE ACQUIREMENT OF CHARACTERS ILLUSTRATED. 255 



an ancient type of animals, and expressing great persistence 

 in some lines of succession, they also present very clear indi- 

 cations of definite succession and limit in their generic life- 

 periods, as may be expressed again numerically in the follow- 

 ing way : To express this law we may select a group of re- 

 lated forms, grouped under three families by Schuchert, the 

 Terebratulidce, the Dyscoliida y and the Terebratellidce, in which 

 are known 66 genera. Of these genera 13 were initiated in 

 the Paleozoic (i Ordovician, I Silurian, 5 Devonian, 6 Car- 

 boniferous) according to present statistics, 38 in the Meso- 

 zoic (7 Triassic, 23 Jurassic, 8 Cretaceous), and 10 are known 

 only in the Recent. Of these 66 genera 41, or about f, 

 have a recorded continuance of only one era, 1 1 are recorded 

 from two contiguous eras, 2 from three, 4 from four, 4 from 

 five consecutive eras. 



Climax of Generic Evolution at a Definite Period, If we go 

 one step further, and analyze the range of the genera of a 

 single subfamily, we see the law of evolution expressed with 

 greater clearness. Using Schuchert's list of genera, we find 

 that the subfamily of Dallinince (the first division of the 

 family Terebratellidae) contains 22 known genera; all of these 

 are known not earlier than the first period of the Mesozoic. 

 Of them, 3 genera are first seen in the Triassic, 13 first in the 

 Jurassic, 2 first in the Cretaceous, I first in the Tertiary; 3 

 are only known as recent species. In this case it is perfectly 

 evident that the group is a Mesozoic type, that it began to 

 appear in numbers in the Triassic, that its greatest expansion 

 was in the Jurassic, that as a subfamily it is not now extinct. 



We draw from these facts the conclusion that there was 

 constant evolution going on, that all along geological time 

 old types were dying out and new ones were being initiated 

 or introduced. It is by studying the characters expressed by 

 these successive genera, and noting their relation to each 

 other in the order of their succession, that we catch a glimpse 

 of the actual facts of evolution as they have taken place in 

 the past. 



In order graphically to express the grand facts of the 

 evolutional history of the various types of Brachiopoda the 

 following diagram of the evolution-curves of the various divi- 



