26o GEOLOGICAL BIOLOGY. 



that he would notice; for the ordinary observer will notice 

 some of the specific characters more readily than he will the 

 more essential characters of a strange object. 



Before leaving the first era of our time-scale we have 

 still more characters of family rank, others of subfamily rank, 

 and enough elaboration of them to call for classification into 

 fifty-five genera of Lyopomata and five genera of Arthropo- 

 mata, and all these are found in the species now known from 

 Cambrian rocks. 



On the theory that the organisms now living are de- 

 scended from ancestors in the past, the characters once hav- 

 ing appeared in the ancestral line are most simply accounted 

 for by supposing that they have been transmitted without 

 change by the laws of ordinary generation. However the 

 characters may have been originally produced, or came 

 about in the first place, having once appeared in the Cam- 

 brian their continued reappearance in later stages of geologi- 

 cal history calls for no other processes than those we see 

 taking place on every hand, i.e., the successive reproduction 

 of offspring by regular generation : no action of evolution is 

 required. The preservation by continued generation of these 

 ancient ancestral characters is no less remarkable than the 

 slight modifications which have taken place in the course of 

 geological ages. 



Evolution Accounts for Divergence, not for Perpetuation or 

 Transmission. — This familiar law of heredity will account for 

 the continuance, as long as they appeared, of the families and 

 genera of the Cambrian ; the appearance of new families 

 new genera, and new species requires on this theory the 

 assumption of some other process. When we examine the 

 length of recurrence of these Cambrian forms, of them we 

 find only three genera of the Lyopomata and one genus of 

 the Arthropomata are known from rocks above the Cambrian, 

 and they are from the next succeeding system. Of the 

 family characters of the Cambrian Brachiopoda, six Lyopo- 

 mata, two Arthropomata families lived after the Cambrian: 

 two of these lived on to the Carboniferous, two of them 

 reached the Silurian, and only three reached the Ordovician. 

 There were, however, four families and two genera that ap- 



