THE CAMBRIAN PERIOD 71 



ditions favorable for the development of higher forms by the 

 aggregation of cells. The plants first spread to the shore waters 

 and thence over the land, so that gradually the shore waters became 

 clearer and richer in organic material and hence more suitable 

 habitats for animals. The animals once established along shore, 

 about the beginning of the Cambrian or late in the pre-Cambrian, 

 are conceived to have made rapid progress in evolution because 

 the struggle for existence became severe on account of greater 

 crowding in this more restricted environment. Support became 

 necessary as well as means of defense, therefore hard parts were 

 developed, and such hard parts could be preserved as fossils. In 

 harmony with this hypothesis is the important fact that pre- 

 Cambrian and early Cambrian fossil shells are mostly very thin, 

 heavy shells apparently not having been evolved till later. 



Another hypothesis " assumes that the first forms of life were 

 simple plants that originated in the land waters. . . . This hy- 

 pothesis further assumes that the early animals, to a greater or 

 less degree, had their origin in the same waters, and like the plants 

 on which they were dependent spread thence to the sea and out 

 upon the land. It is assumed that there might be considerable 

 development of aquatic forms of animal life ... in the land waters 

 before they became denizens of the seas, and their appearance 

 in the latter might be at some rather advanced stage of their 

 evolution and hence be seemingly sudden. " l 



Plants. — There are certain rather obscure impressions and 

 other more distinct cluster-like forms which may be sea-weeds, 

 but their identification is not at all positive. As stated above, 

 simple plants at least must have been abundant since animals 

 ultimately depend upon plants for food. Their scarcity as 

 fossils is doubtless due to the unfavorable character of the simple 

 (soft) marine plants for fossilization. 



Recently certain problematical Cambrian fossils, long known 

 by the name "Cryptozoon," have been determined as Algae by 

 Walcott. They secreted concentric layers of carbonate of lime 

 and lived in water. In some localities, as near Saratoga Springs, 

 New York, distinct beds or " reefs" of such Algae occur in lime- 

 stone (see Fig. 32). 



Protozoans. — Foraminifers have been found even in Lower 

 Cambrian rocks. These forms are very much like the modern 

 1 Chamberlin and Salisbury: Geology, Vol. 2, p. 302. 



