354 PROCEEDINGS OF SECTION C. 
Rate of Variation.—When we think that certainly seven, and 
probably all eight, of the sub-kingdoms of animals were in existence 
before the close of the Cambrian period, it would seem at first that 
variation had gone on more rapidly during the earlier periods of 
the earth’s history than afterwards; but this is an erroneous 
impression, due to the very unequal lengths of time represented 
by the different periods. Making every allowance for tke possi- 
bility that the rate of denudation and depression may have been 
greater in past times than now, still we must admit that the 
relative thickness of the sedimentary rocks of each period is a 
rough measure of the relative length of time it represents ; and I 
suppose that every geologist will agree that the Huronian, the 
Algonkian, the Cambrian, and the Ordovician were collectively at 
least equal in duration to all the periods that came after them— 
that is, they represent at least one-half of the time since life first 
appeared on the earth; but certainly the changes which have 
taken place in animals, and especially in plants, since the com- 
mencement of the Silurian period, are far greater than those that 
went before, both in the addition of new groups and in the 
extinction of old ones; so that the rate of variation must have 
increased and not diminished with time. It was this slow rate of 
variation in ancient times that enabled the early Paleozoic genera 
to spread so much more widely over the earth than do the genera 
of the present day. 
Hatinction of Growps.—The diminution or decay of a whole 
group of animals first began with the Graptolites in the Upper 
Ordovician, and they finally became extinct in the Carboniferous. 
The same process commenced with the Trilobites in the Silurian 
period, and they became extinct in the Permian. Can we trace 
any cause for this gradual process of decline in numbers? The 
existence in the earliest times of Radiolarians almost identical 
with their descendants of the present day is but another example 
of the persistence of types with which paleontologists have been 
familiar for a long time. It is true that we only know the hard 
parts of the ancient forms, but we have reason for thinking that 
if the soft parts had varied much the hard parts would have 
changed also. From the fact of the persistence of certain types 
it necessarily follows that there is no inherent necessity for 
organisms to vary or to decay, while the idea that if they vary 
then they must subsequently decay is opposed to the whole 
teaching of organic evolution, for itis the variable groups which 
have progressed. But if there is no internal necessity for decay, 
then the extinction of a whole group must be due to external 
agencies, and, if the group is widely spread, these agencies cannot 
have been local in their operation. 
These external agencies may be changes in climate, or changes 
in the biological environment, due to the introduction of new 
