THE CAMBRIAN PERIOD 409 



of the animal kingdom all except the vertebrates have representatives 

 in the former. If, as is generally believed, this differentiation was the 

 result of slow evolutional changes, it is probable that a greater length 

 of time was required to produce such a divergence than for all the 

 changes in life that have taken place since the Cambrian. 



Another indirect evidence is found in embryology. Each animal 

 in its development from the egg to the adult condition passes through 

 a series of stages which resemble those through which the race passed 

 in its evolution, many embryonic stages representing those of mature 

 but remote ancestors. It is evident, therefore, that the embryonic 

 and larval stages of the individual furnish somewhat of a basis upon 

 which to estimate the length of the evolutional history of the race 

 to which the individual belonged. Some of the larval stages of the 

 trilobites are preserved and give firm ground for the belief that this 

 class had a long line of ancestors previous to the Cambrian. 



Plants 



Since all animals depend directly or indirectly upon vegetation for 



their food, it is evident that plants must have been in existence in 



large numbers in the Cambrian in order to supply 



with food the abundant marine animal life of 



that time. When, however, a search for plant 



fossils is made, none are found that can with 



certainty be recognized as plant remains. The 



inference is forced upon us that Cambrian plants 



were not highly organized, and that they possessed 



s ^ & . ' y F Fig. 378.— Aprob- 



little or no woody tissue and were, consequently, lematical fossil, Old- 



incapable of fossilization. Some poorly defined, hamia antiqua, which 



stemlike impressions found in the Cambrian has .!> eei V some ^ Imes 



considered to be a 

 strata at Burlington, Vermont, and elsewhere p i an t. 



strongly suggest the stems of seaweeds, but some 

 of these may be worm tracks ; some, rill marks ; and some, trailings 

 made by animals. The difficulty in determining such " fossils " is 

 well shown in the controversy over the determination of certain 

 specimens (Oldhamia) found in the Cambrian rock of Ireland, which, 

 as the illustration shows (Fig. 378), have the appearance of vege- 

 table growth. Some investigators have classed them as the remains 

 of animals, some as plants, and some as inorganic markings. 



The scarcity of plant fossils may perhaps be attributed to the fact 

 that only Cambrian rocks of marine origin have been studied, since 



