674 Geological Periods of Mutation. 
1. The number of elementary characters of a higher 
plant, that is to say, the number of mutations through 
which its ancestors have passed from the beginning, is 
probably not more than a few thousand. 
2. The average intervals of time between two suc- 
cessive periods of mutation are similarly to be estimated 
at a few thousand years. 
3. From this we may conclude that a period of some 
millions of years is sufficient for the whole development 
of the animal and vegetable kingdom ; or, in other words, 
4. The doctrine of mutation does not demand a longer 
period for the duration of life than that which has been 
given by LORD KELVIN, viz., 24 million years. 
These theses may be most simply summarized in the 
statement that the product of the number of elementary 
characters of an organism, and of the mean interval of 
time between two successive mutations of its ancestors, 
is equal to the extent of biological time. 1 If we call the 
former magnitude M (the number of mutations), the 
length of the intervals L and biological time BT, we have 
the expression 
M X L = BT. 
I have called this the biochronic equation. 2 It will, 
I hope, help to demonstrate the importance of the doc- 
trine of the elementary units of organisms, and thus 
bring this doctrine prominently before the eyes of the 
general reader, as well as of the trained investigator. 
This is my main object in enunciating it. 
1 The mutations referred to in these paragraphs are of course 
progressive mutations. 
~Dic Mutationen und die Mutationsperioden, p. 63. 
