670 Geological Periods of Mutation. 
the drawings on Roman coins, and many other facts of 
the same kind conduce to similar estimates. 1 On the 
other hand, the rarity of mutable plants in comparison 
with immutable 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 floras, 
lead by an entirely different chain of argument to con- 
clusions which mainly support those reached above. 
We may therefore assume as a provisional conclu- 
sion that a few thousand years elapse on the 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. 2 On the other hand there must 
have been periods of greater mutability and periods of 
relative stagnation, possibly in 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. The 
Cambrian period divides biological time into two ap- 
proximately equal parts , no fossil remains from pre- 
Cambrian times are known In Cambrian times members 
of all the more important groups of invertebrates sud- 
denly appear, and among plants the Algae are richly 
represented. It almost seems that only those lines of 
descent which have made their evolution on the con- 
tinents have begun in post-Cambrian times. 
In a very attractive essay BROOKS has shown how 
1 Instances of the ages of certain plants are given by DE CAN- 
DOLLE, Geographic botaniquc, II, 1063-1068, 1086 etc. 
2 On this point see my lecture cited above, pp. 52-57. 
