THE AVONIAN OF THE AVON GORGE 
83 
tremely valuable measurement of time-intervals ; for, in such 
cases, the variation-gradient is abnormally large and appreciable 
variation takes place in small intervals of time. Among the 
Corals, the Clisiophyllid group exhibits this rapid variation in 
the most striking manner and the degree of specialization 
affords a valuable guide to zonal position. 
There is one other natural law which assists very materially 
in the determination of time-level. 
At any one period, there is a general tendency among the 
gentes of the different genera of the same class to adopt some 
particular character which is possessed by the particular group 
of that class which is dominant at the time. 
For example in Dihunofhyllum-timQ, (1) the Cyatho'phylla and 
the Lithostrotions adopt, more or less completely, a Clisiophylli- 
dan type of structure. 
(2) Productus aff. giganteus and Chonetes aff. comoides become 
closely assimilated. 
In the working of this law, genera lose their distinctness and 
their artificiality is often strikingly displayed. 
Characters which are common to several genera at the same 
time may be termed time-traits, and the general law itself may 
be entitled convergence or coeval assimilation. 
To sum up : — 
A natural system of zonal indices must be based upon the 
variation of the gens, those gentes being best adapted for our 
purpose whose variation-gradients are large. 
In practice, dominant species must be relied upon for purposes 
of zoning and, consequently, we are frequently forced to change 
from one gens-line to another. 
The presence of a zonal fossil suggests a zone, but does not 
fix it ; for it is, by nature, impossible to select a series of indices 
which shall be exactly conterminous. 
