EVOLUTION OF FOSSIL CEPHALOPODA. 23 1 



If the geologic record is incomplete, the biologic 



record is still more so ; the beginnings of things are 



lacking just where we should most like 

 Incompleteness ^.^ ^^^ ^^^^ gj^^^^. ^^^^ ^f ^^^ branch- 



of the record. . , . 1 , ^ 1 • 



ing stocks were mcapable of bemg pre- 

 served as fossils, or we have yet to find the strata in 

 which they may be preserved. In the Cambrian the 

 divergence was already complete, all the subkingdoms 

 were present except the vertebrates, and they, too, 

 probably existed at that time, for highly specialized 

 placoderm fishes have been found in the Lower Silurian. 

 The brachiopods had already branched out into articu- 

 lates and inarticulates, the molluscs into pelecypods, 

 gastropods, and cephalopods. The crustaceans were 

 already represented by phyllocarids, trilobites, and 

 ostracods, widely divergent types. Even at the base of 

 the Cambrian beds all these animals, especially the 

 trilobites, went through many larval changes before they 

 reached maturity, thus indicating a long family history 

 of numerous pre-existing unknown ancestral genera. 



LAW OF ACCELERATION OF DEVELOPMENT. 



Since the geologic record is so badly broken, and 

 since modern faunas and floras are but the topmost 

 branches of a tree whose stock is only partly known, the 

 early naturalists were merely groping in the dark in 

 their efforts to get a natural classification. There was, 

 however, a glimmer of light, although scarcely heeded. 

 No one man seems to have been the discoverer of the 

 law of acceleration of development, but, like the idea of 

 evolution, it was in the air, and disclosed itself in 

 various ways to the prophetic vision of seekers after 

 truth. J. F. Meckel,* a German naturalist, seems to 

 have been the first to give scientific expression to the 



* Syst. Vergl. Anat., i, Theil, Halle, 1821. 



