26 
ORGANIC DEPENDENCE AND DISEASE 
had the longer life, its type was locked up almost from the 
start, and except for the lesson it teaches of stagnation and 
decline, we might say, without impiety, that its conservation 
has been a waste of time. And it is a type, too, that was 
won, not by the arduous struggles of the ages, but arrived 
at early and with ease. Therefore its lessened worth. 
DIVISIONS OF GEOLOGICAL HISTORY 
We cannot well proceed with this discussion without a 
succinct statement here of the stages of geological history, 
in which special emphasis is laid upon those earlier di- 
visions with which we are especially concerned. The table 
that follows is a condensed one of standard acceptance ; it 
begins at the top of the latest life-bearing rocks and ends 
with the oldest. As to the estimates of time represented 
for the deposition of these sediments and for the existence 
of the life of the earth, this must be said: Ten years 
ago there was considerable variance of opinion be- 
tween the physicists who were estimating the age of the 
planet on the basis of the external disturbances to which it 
was subjected in our planetary system, and the geologists 
who sought to approach this problem from measurements 
of the rate of deposition and erosion of water-laid sedi- 
ments ; but a conservative conclusion had been provisionally 
attained which was tacitly accepted by most geologists as 
somewhere between sixty and one hundred million years 
for the sum of all water-laid rocks and perhaps from forty 
to sixty million years for those rocks which still carry the 
obvious remains of life. Since the discovery of radium and 
with a growing understanding of the significance of radium 
decomposition- and radio-activity these estimates have been 
enormously outstripped, so vastly indeed that the very size 
of the figures seems to put them under suspicion. The time 
element in this is still a factor of much discussion and 
