ENVIRONMENTAL CONDITIONS 397 



deposits rich in carbon, but the low altitude of lands and the wide spread 

 of shallow seas reduced terrestrial deposits to a minimum. The same 

 conditions apply to the Middle and Upper Cambrian. Thus, until the 

 opening of the Devonian, our knowledge of the life of the lands is pecu- 

 liarly meager in comparison to our knowledge of the life of the sea. 



Let us turn to another line of argument. The zone of brackish water 

 is restricted to discontinuous embayments. The nature of the waters is 

 transitional between those of sea and land. It is a habitat, consequently, 

 which is not the center of dispersal for any class of organisms, but its 

 faunas have greater relationships to the sea than to the land. Where 

 brackish embayments occur in regions of deposition, they are most com- 

 monly marginal to deltas and result from the shifting struggle between 

 land and sea. In a dry season the salinity of some embayments may be 

 higher than that of the open ocean. In the season of flood the river 

 distributaries may rise or shift and inundate these bays with mud and 

 fresh waters. It is only the hardiest of organisms, consequently, which 

 can withstand the vicissitudes of these regions. On the one hand, ccelen- 

 terates, echinoderms, and cephalopods especially avoid the shifting sands, 

 smothering muds, and changing waters of such habitats. Mollusks, 

 arthropods, and vertebrates, on the other hand, show a more ready adapta- 

 tion to these conditions, and it is from these phyla especially that the 

 brackish waters are peopled. But the faunas are derivatives either from 

 permanent marine or permanent fresh-water stocks. If, then, the fishes 

 of these brackish embayments came from the sea, why are not their fossils 

 found in greater variety, abundance, and perfection of preservation in the 

 much wider, more continuous, and organically richer deposits of the 

 epeiric seas ? The answer which is indicated is that the primitive fishes 

 were not there. Their theater of evolution was elsewhere. Only a few 

 pioneer species, or the bodies of dying individuals, or their water-worn 

 and scattered plates and scales, were washed out to, sea. They were, in 

 Ordovician and Silurian times, creatures of the land waters. The out- 

 skirts of their habitat were in the brackish embayments. In such em- 

 bayments or in near-by marine waters the chances of geological preserva- 

 tion were, however, increased. The recurrence of dermal plates and 

 spines in the deposits of such regions show that they possessed hard parts 

 capable of good fossilization. In contrast to this meager showing of 

 ostracoderm plates, scattered teeth of unknown affinities and elasmobranch 

 spines, which constitute the earlier record, is to be noted the truly marine 

 record, which begins with the opening of the Middle Devonian and be- 

 comes more abundant in the Lower Carboniferous. This expansion in 

 marine fossils is the mark of a real invasion and conquest of the sea, not 



