THE EARLIEST ARACHNIDA 41 



of the decline rather than the rise of a group. No 

 progressive development, but a gradual or rapid ex- 

 tinction, and consequent reduction In the number of 

 genera and species, is a summary of the record of the 

 fosslliferous rocks as regards this group and many others, 

 such as the Trilobltes, the Brachiopods, and the Nautili- 

 dae. All these groups begin with many forms In the 

 oldest fosslliferous rocks, and three of them have left 

 genera practically unchanged from their first appearance 

 to the present day. What must have been the time 

 required to carry through the vast amount of structural 

 change Implied In the origin of these persistent types 

 and the groups to which they belong — a period so ex- 

 tended that the Interval between the oldest Palaeozoic 

 rocks and the present day supplies no measurable 

 unit ? 



But I am digressing from the Appendlculate Phylum. 

 We have seen that the fossil record is unusually com- 

 plete as regards two Classes in each grade of the 

 Arthropod branch, but that these Classes were well 

 developed and flourishing in Palaeozoic times. The 

 only evidence of progressive evolution is in the develop- 

 ment of the highest orders and families of the Classes. 

 Of the origin of the Classes nothing is told, and we 

 can hardly escape the conclusion that for the developm.ent 

 of the Arthropod branches from a common Chaetopod- 

 like ancestor, and for the further development of the 

 Classes of each branch, a period many times the length 

 of the fosslliferous series is required, judging from the 

 insignificant amount of development which has taken 

 place during the formation of this series. 



It is Impossible to consider the other Coelomate Phyla 

 as I have done the Appendiculata. I can only briefly 

 state the conclusions to which we are led. 



As regards the Molluscan Phylum, the evidence is 

 perhaps even stronger than in the Appendiculata. Re- 

 presentatives of the whole of the Classes are, it is 

 believed, found in the Cambrian or Lower Silurian. 

 The Pteropods are generally admitted to be a recent 

 modification of the Gastropods, and yet, if the fossils 



