THE CAMBRIAN PERIOD 503 



pression of the amount of evolution already accomplished. If to 

 this be added a comparison between the Cambrian life and that of 

 the present time, an estimate of the relative amount of evolution 

 before and since the Cambrian period may be made. 



It is to be noted that not only were all the animal sub-king- 

 doms, save perhaps the vertebrate, present, but that, in many of 

 them, the forms had come to have so nearly the aspect of living 

 forms that the classes and some orders are readily recognized. 

 The initiation and divergence of ike structures and types that preceded 

 the Cambrian stage mean much more in the way of evolution, than all 

 the evolution of later times. Formulated in numerical terms, we 

 may perhaps say that 60 to 90 per cent of the evolution represented 

 by the life of today, had been accomplished in pre-Cambrian times. 



Mental development. The wars' of the Cambrian animals, 

 implied by their weapons of offense and defense, can scarcely have 

 been unaccompanied by some notable measure of mental develop- 

 ment, however the nature of such development may be interpreted. 

 That the trilobites sought their food or pursued their prey by sight, 

 and were guided by touch, is implied by their eyes and antennae, 

 and it is difficult to conceive of functional senses without the 

 mental processes that usually attend pursuit and capture. 



Ecological adaptations. The distribution of the Cambrian 

 fossils indicates that then, as since, there was an adaptation of life 

 to its physical environment. There seem to have been zones of 

 shore life, off-shore life, and deep-sea life, although the evidence of 

 the last is scant. These variations must be taken into account 

 in the comparison and correlation of faunas, for considerable geo- 

 graphic differences may occur among faunas which were strictly 

 contemporaneous . 



Zoological provinces. The assemblages of the life of the period 

 seem to have varied in a broader way, giving rise to zoological 

 provinces. It is probable that the leading factors in the develop- 

 ment of these provinces were barriers that isolated, or partially 

 )lated, certain portions of the sea from other portions. The 

 separation must have reached such a degree as to cause the life 

 of each area to develop along its own lines, in more or 3ss indepen- 

 lence of the evolution of other regions. 



