THE CAMBRIAN PERIOD. 211 



of development of the various organic structures and functions, and 

 (2) by the amount of divergence of the animal types. 



(1) For comparison it may be assumed that the primitive forms 

 were at least as simple as the simplest existing forms. Among these 

 there are organisms which are scarcely more than aggregates of living 

 protoplasm, almost devoid of any permanent and recognizable organs. 

 There are also multitudes of plants and animals that consist of a single 

 cell only. If these be taken as representing the nearest existing approach 

 to primitive forms, a comparison between them and the complicated 

 structures shown by many of the Cambrian fossils, presently to be 

 described and illustrated, will give some impression of the degree of 

 advance in organization that had been attained. 



But we are not left entirely to this mere presumption that the 

 earliest forms were much simpler than the Cambrian, for the stages 

 of development of the young of certain of the Cambrian animals reveals 

 something of their ancestral history. It is a well-established law of 

 embryology that animals in their pre-natal and youthful development 

 pass through a succession of stages in which their structure resembles 

 that which their ancestors had in their maturity; in other words, that 

 the individual history of each animal is an epitome of the collective 

 history of its ancestors. Now the trilobites (Fig. 118), the leading form 

 of Cambrian life, are known to have passed through a series of quite 

 remarkable changes after they became well enough developed to be 

 fossilized, and doubtless they passed through other stages previously. 

 There is, therefore, specific ground for* believing that they had a long 

 line of ancestors. 



(2) The studies of recent decades have convinced investigators that 

 the later forms of life have been derived from earlier ones by some 

 process of evolution. The exact nature of the process is yet under 

 investigation, but the fact of derivation is not now regarded as an open 

 question. Ao the various forms developed and diverged, many of the 

 intermediate gradations were dropped out, because of inferior fitness, 

 or from some other cause, and thus the diverging forms became sepa- 

 rated from one another. As these divergent branches themselves 

 developed later, they in turn diverged, and the intermediate forms 

 disappeared, and thus a succession of branches was developed. By 

 such continued divergence and loss of intermediate forms, a more 

 and more complicated system of branching was developed. Not only 



