THE ACQUIREMENT OF CHARACTERS ILLUSTRATED. 26 1 



peared in the second era, the Ordovician, which Hved on to 

 the present time, and it is not improbable that these types 

 of differentiation may have taken place as early as the Cam- 

 brian. 



Brachiopods Ancient Types and Early Differentiated. — From 

 these facts we learn that the Brachiopods are very ancient 

 animals; that at the first geological period they were very 

 greatly differentiated in structure, and that, except in a very 

 few cases, the forms that lived in later ages, though suppos- 

 edly descended from the earliest types, suffered changes in 

 their specific, generic, and in many cases family characters. 



It is also evident that if we wish to study the history of 

 Brachiopods we must read the evolution in terms of their 

 specific, generic, and only in slight degree in any characters 

 of as high rank as family, and not at all in characters of 

 higher than family rank. 



A glance at the range of the families and genera of the 

 Lyopomata shows them not only to have been ancient, but to 

 have reached their climax of evolution by the second geologi- 

 cal period of time — the Ordovician. After the Ordovician no 

 new families of Lyopomata are initiated, and the new genera 

 fell from twenty-two new ones in the Ordovician to three in 

 the Silurian, six in the Devonian, and after that seven new 

 genera up to the living forms. This slight continuance of ex- 

 pansion may be driven much farther back by later discov- 

 eries. 



Laws of Evolution Gathered from Study of the Early Families. 

 — With such an early expansion of the suborder it is evi- 

 dent that the range of instructive history is limited to the 

 earliest periods of geological time, and the few forms 

 that still exist among the recent faunas are very slightly 

 modified from the ancient types. In the case of the other 

 suborder, Arthropomata, the evolution was continued to a 

 later period. Family and subfamily differentiation was 

 greatest in the first two geological eras, nine new families 

 appearing in the Ordovician; but two or three new genera 

 in each of the following eras, except in the Cretaceous and 

 Tertiary, when the present information records only a single 

 new subfamily in each. 



