550.1 327 
560 
LY. 
The Imperfection of the Geological Record 
T is now many years since Darwin first directed the special 
attention of biologists to the imperfection of the geological 
record. It was he who first satisfactorily marshalled the facts which 
prove that the discoverable fossils in the rocks can only give a 
very limited idea of the plants and animals which have tenanted 
the globe at different periods in its past history. -He pointed out 
how small a portion of the earth had been geologically explored, 
and how small a percentage of known types of life had sufficient 
hard parts to be preserved in a fossilised state. He emphasised the 
fact that the number both of specimens and of species preserved in 
our museums, is absolutely as nothing compared with the number of 
generations which must have passed away even during a single 
geological formation. He also observed “ that, owing to subsidence 
being almost necessary for the accumulation of deposits rich in 
fossil species of many kinds, and thick enough to outlast future 
degradation, great intervals of time must have elapsed between 
most of our successive formations; that there has probably been 
more extinction during the periods of subsidence, and more variation 
during the periods of elevation, and during the latter the record 
will have been least perfectly kept ; that each single formation has 
not been continuously deposited” ; that, indeed, in every area of the 
earth’s surface there are incalculable periods of geological time 
unrepresented in the records of the rocks. 
’ We may, in fact, without exaggeration declare that every item of 
knowledge we possess concerning extinct plants and animals depends 
upon a chapter of accidents. Firstly, the organism must find its 
way into water where sediment is being deposited and there escape 
all the dangers of being eaten ; or it must be accidentally entombed 
in blown sand or a volcanic accumulation on land. Secondly, this 
sediment, if it eventually happens to enter into the composition of 
a land area, must escape the all-prevalent denudation (or destruc- 
tion and removal by atmospheric and aqueous agencies) continually 
in progress. Thirdly, the skeleton of the buried organism must 
resist the solvent action of any waters which may percolate through 
the rock. Lastly, man must accidentally excavate at the precise 
spot where entombment took place, and someone must be at hand, 
