THE CAMBRIAN PERIOD. 293 



the close of the period, as the seas crept out upon the face of the con- 

 tinents, there was a marked tendency toward cosmopolitanism. The 

 impression that life was everywhere the same at a given stage in the 

 early ages, which naturally arises from the very general and limited 

 treatment of the faunas to which works of this kind are confined by 

 their limitations of space, is to be guarded against. There was prob- 

 ably less variation in the life of the different regions, during most of 

 the geological epochs, than there is to-day, but it is not certain that 

 this was true of all past epochs, and in none of them was there prob- 

 ably anything like complete uniformity over the whole globe, though 

 there was, at times, much tendency in that direction, as implied by 

 the term cosmopolitanism. 



Comparison of pre-Cambrian and post-Cambrian evolution. — After 

 this summary review of the Cambrian life, it is appropriate to return 

 to the question raised at the outset relative to the degree of develop- 

 ment which this first well-preserved fauna exhibits. On the anatomi- 

 cal and physiological side, it is clear that nearly or quite all the funda- 

 mental organs had been developed. There were skeletal systems 

 of several forms ; there were muscular systems, as shown by the impres- 

 sions and protrusions on shells and as implied by the articulate struc- 

 ture of the trilobites; there were nervous systems, as implied by eyes 

 and other evidences of sense-organs, and by the need for muscular 

 control; there were organs for capturing and ingesting food, and 

 hence, almost certainly, organs of digestion, secretion, excretion, and 

 respiration; in short, there were practically all the great anatomical 

 and physiological systems now possessed by animals. The Cambrian 

 animals had acquired the various modes of life possessed by existing 

 animals of their kind, as well as the various modes of preserving their 

 lives. It is probable that all, or nearly all, the senses had some develop- 

 ment, though this cannot be proved for certain senses. The eyes of 

 the trilobite show that that wonderful organ was already developed 

 in a notable degree. 



In the evaluation of these evolutions it is not to be overlooked that 

 the initiation of all these structures and functions is involved. 



In the matter of the divergence and the isolation of classes, and 

 the establishment of type forms, it is to be again noted that not only 

 were all the animal subkingdoms, save perhaps the vertebrate, pres- 

 ent, but that, in many of them, the forms had come to have so nearly 



