The Biochronic Equation. 665 
ment far better than that theory. We now see that when 
applied to the great problems of life it is free from 
those insuperable difficulties, which so many investigators 
have found to stand in the way of the theory of selec- 
tion. 
The theory of selection demands almost unlimited 
time for the evolution of organisms; for the mutation 
theory, on the other hand, the time which the physical 
geologists grant to life, is amply sufficient. This view 
was first clearly expressed by BROOKS, in accordance with 
HUXLEY, when he showed that all the difficulties which 
beset the theory of selection and which, according to 
many investigators, needed 2,500,000,000 years for the 
whole process of evolution, would disappear if we assume 
relatively sudden and discontinuous changes to take place 
from time to time. 1 
The most distinguished investigators demand a period 
of about 24 million years, to cover the duration of life 
on the earth. If the ancestors of our Oenothera La- 
marckiana have produced, once in every 4000 years, a mu- 
tation which made them richer by a single character, our 
plant would now be composed of 6000 such characters, 
a number far higher than comparative and systematic 
science can by any means accumulate in its description. 
This rough calculation shows at any rate that the 
demands made by the theory of mutation are not so exor- 
bitant as those made by that of selection. Neither the 
number of mutation periods passed through, nor that of 
the characters acquired in them, is beyond our powers 
of comprehension. On the contrary the phenomena 
viewed from this standpoint are such that they indicate 
the possibility of a much closer investigation. 
1 W. K. BROOKS, loc. cil., p. 286. 
