THE CAMBRIAN PERIOD 501 



most significant feature of the protective devices lies in the fact that 

 they are usually of the same type as those possessed by the similar 

 animals of later times. The shells of the gastropods, pelecypods, 

 and brachiopods differ from thoss of to-day in minor features only. 

 The coverings of the trilobites were much like those of their living 

 relatives, and much the same may be said of nearly every form. 

 If there had been a radical change in the character of their enemies 

 or rivals, we might expect some notable change in the defensive 

 devices. It is a natural inference, therefore, that the conflicts of 

 life in the Cambrian seas were similar to those of the present time. 

 The inference may be pushed a step further, and the deduction 

 drawn that the conflicts which led to the evolution of the defensive 

 devices were much like those throughout the period of their reten- 

 tion. 



Stage of evolution represented. What stage of advancement in 

 the development of life had been already attained by the begin- 

 ning of the Cambrian period? Do the fossils of the system indicate 

 that the life of the period was primitive, or do they imply that it 

 had advanced far beyond primitive forms? The answer to these 

 questions is to be sought (1) in an estimate of the degree of devel- 

 opment 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 of life were as simple as the simplest existing forms. There 

 are multitudes of plants and animals that consist of a single cell, 

 and if these are taken as representing the nearest existing approach 

 to primitive forms, how far had the Cambrian life advanced beyond 

 them? 



We are not left entirely to the presumption that the earliest 

 forms of animal life were much simpler than those of the Cambrian, 

 for the stages of development of the young of certain of the Cam- 

 brian animals reveal something of their ancestral history. It is 

 a well-established law of embryology that animals, in the early 

 stages of their development, pass through a succession of changes 

 in which their structure resembles that which their ancestors had 

 in their maturity; in other words, the individual history of any 

 animal is an epitome of the history of its ancestors. Now the 



