670 Geological Periods of Mutation. 



the drawings on Roman coins, and many otlicr facts of 

 the same kind conchice to similar estimates.' On the 

 other hand, the rarity of mutable plants in comparison 

 with immntal)le ones, and also the small number of genera 

 and other groups rich in species, as compared with the 

 ordinary types of the European and American Roras, 

 lead bv an entirely different chain of arqument to con- 

 elusions which mainly support those reached above. 



We may therefore assume as a provisional conclu- 

 sion that a few thousand }-ears elapse on tlie average 

 between two successive periods of mutation. Of course, 

 it is extremely probable that the speed of the process of 

 evolution has not at all times been the same. On the 

 one hand we must suppose that at first it was more rapid 

 than it is at present.- On the other hand there must 

 have been ])eriods of greater mutability and periods of 

 relative stagnation, ]:)()ssibly m the whole animal and vege- 

 table kingdom, but certainly in special lines of descent 

 owing to which some have reached a high degree of 

 differentiation in the same period of time in which the 

 progress in other lines has been relatively small. 1die 

 Cambrian period divides biological time nito two ap- 

 proximately e(|ual parts, no fossil remains from pre- 

 Cambrian times are known In Cambrian times members 

 of all the more important groups of invertebrates sud- 

 denly a])pear, and among plants the :\lgae are richl\' 

 represented. It almost seems that only th(^se lines of 

 descent which have made their evolution on the cdu- 

 tinents have begun in post-Cambrian times. 



In a very attractive essay Brooks has shown how 



^ Instances of the ages of certain plants are given by De Can- 

 DOLLE, Geographic hotaniquc, II, 1063-1068, 1086 etc. 



"On this point see my lectnre cited above, pp. 52-57- 



