LIFE 363 



Sponges lived throughout the period. It is probable that many 

 protozoans existed, but only a few forms have been identified. 



Implied life. The existence of so much animal life implies 

 much vegetable life to supply the necessary food. Furthermore, 

 various characteristics of the fossils suggest the presence of animals 

 not known from fossils. A large percentage of the known Cambrian 

 animals were provided with shells, tests, plates, or other forms of 

 hard coverings. In the main, these appear to have been protective 

 devices, and imply enemies or rivals against which protection was 

 needed. Perhaps the most significant feature of the protective 

 <lr vices is that they are of the same types as those possessed by 

 similar animals of later times. 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. The inference may be pushed 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 retention. 



Stage of evolution represented. What stage of advancement 

 in the development of life had been attained by the beginning 

 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? For comparison it 

 may be assumed that the first forms of life were as simple as the 

 simplest existing forms. If the plants and animals that consist of 

 a single cell are taken to represent primitive forms, how far had 

 the Cambrian life advanced beyond them? 



In the early stages of their development, animals 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 Cambrian trilobites are known to have passed through a 

 M'rirs of remarkable changes after the individuals had developed 

 far enough to be fossilized, and it is inferred they passed through 

 other stages previously. There is, therefore, specific ground for 

 believing that they had had a long line of ancestors. 



On the anatomical and physiological side, it is clear that nearly 

 or quite all the fundamental organs had been developed. There 

 were skeletal systems of several forms, muscular systems, nervous 



