THE ACQUIREMENT OF CHARACTERS ILLUSTRATED. 263 



differentiation shows such irregularity that we are led to sus- 

 pect within it three well-defined and separate courses of evo- 

 lution, one of which culminated in the Silurian, one in the 

 Carboniferous, and the third in the Jurassic. The presump- 

 tion is that this group is not well arranged ; the classification, 

 will need revision. 



The third group is the Rostracca, and this is characterized 

 by having a very long geological range; the chief family is 

 the Rhynchonellidae, which appears to extend from the Cam- 

 brian to the present, with its characteristic family-characters 

 the same. 



The fourth group is that of the Helicopegmata of Waagen. 

 This includes the spire-bearing Brachiopods; the history of 

 this group is well defined in families and in genera. The 

 culminating point for both is in the Devonian, when the total 

 number of forms is considered; but the greatest evolution of 

 families is in the Silurian, new genera continuing to appear 

 up to the end in the Jurassic. 



The fifth group includes the AiicylobracJiia. Although 

 the first family of the group appeared in the Ordovician, the 

 evolution of this type was very slow, but continuous to the 

 very end in recent times. From its first appearance each suc- 

 ceeding era has seen the addition of a new family. The curve for 

 generic differentiation is also emphatic, but it shows the evo- 

 lution of this type of Brachiopods to have been late in geolog- 

 ical time. Instead of being in the Paleozoic, the culmination 

 of generic differentiation was in the Jurassic, when twenty- 

 three new genera made their first appearance. The first four 

 family groups of the Arthropomata had their culmination in 

 the Paleozoic, and the fifth had its culmination near the 

 middle of the Mesozoic. 



Conclusions from Study of Generic Evolution Curves of the 

 Brachiopods. — The examination of these differentiation or 

 evolution curves of the generic and family life-histories of the 

 Arthropomata can leave no doubt in our minds on a few im- 

 portant points: 



I. The geological record, although imperfect, and not at 

 all exhaustive in its declarations, reveals the fact that some 

 types of organisms lived in one geological era, others in 



