EVOLUTION OF FOSSIL CEPHALOPODA. 231 
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 
tosee them. Either most of the branch- 
ing stocks were incapable of being 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. 
Incompleteness 
of the record. 
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. 
